View source for Dark matter

{{pp|small=yes}}
{{Short description|Hypothetical invisible cosmic material}}
{{hatnote group|
{{Other uses|Dark Matter (disambiguation)}}
{{Distinguish|Antimatter|Dark energy}}
}}{{Use dmy dates|date=September 2019}}
<!-- Before changing percentages, please note that 85% here refers to that of matter /excluding/ dark energy: (Dark matter 26.8%, Dark energy 68.3%, Ordinary matter 4.9%, Total 95.1% is Dark matter & Dark energy)-->
{{unsolved|physics|What is dark matter? How was it generated?}}
{{Physical cosmology|comp/struct}}In [[astronomy]] and [[cosmology]], '''dark matter''' is an invisible and hypothetical form of [[matter]] that does not interact with [[electromagnetic radiation]], including [[light]]. Dark matter is implied by [[gravity|gravitational]] effects that cannot be explained by [[general relativity]] unless more matter is present than can be observed. Such effects occur in the context of [[Galaxy formation and evolution|formation and evolution of galaxies]],<ref name="Siegfried">{{Cite news |last=Siegfried |first=T. |date=5 July 1999 |title=Hidden space dimensions may permit parallel universes, explain cosmic mysteries |url=http://www.physics.ucdavis.edu/~kaloper/siegfr.txt |work=The Dallas Morning News |archive-date=21 February 2015 |access-date=24 October 2009 |archive-url=https://web.archive.org/web/20150221072439/http://www.physics.ucdavis.edu/~kaloper/siegfr.txt }}</ref> [[gravitational lensing]],<ref name="Trimble 1987">{{cite journal |last=Trimble |first=V. |date=1987 |title=Existence and nature of dark matter in the universe |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=25 |pages=425–472 |bibcode=1987ARA&A..25..425T |doi=10.1146/annurev.aa.25.090187.002233 |s2cid=123199266 |url=https://cloudfront.escholarship.org/dist/prd/content/qt2hz008rs/qt2hz008rs.pdf |archive-url=https://web.archive.org/web/20180718231719/https://cloudfront.escholarship.org/dist/prd/content/qt2hz008rs/qt2hz008rs.pdf |archive-date=2018-07-18 |url-status=live|issn=0066-4146 }}</ref> the [[observable universe]]'s current structure, mass position in [[galactic collision]]s,<ref>{{Cite web |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter |title=A history of dark matter |year=2017}}</ref> the motion of galaxies within [[galaxy cluster]]s, and [[cosmic microwave background]] [[Anisotropy|anisotropies]]. Dark matter is thought to serve as gravitational scaffolding for cosmic structures.<ref >{{cite journal |date=10 October 2023 |title=The Milky Way May Be Missing a Trillion Suns' Worth of Mass |journal=Scientific American |url=https://www.scientificamerican.com/article/the-milky-way-may-be-missing-a-trillion-suns-worth-of-mass/ |archive-date=27 April 2025 |access-date=6 March 2025 |archive-url=https://web.archive.org/web/20250427182021/https://www.scientificamerican.com/article/the-milky-way-may-be-missing-a-trillion-suns-worth-of-mass/ |url-status=live }}</ref> After the Big Bang, dark matter clumped into blobs along narrow filaments with superclusters of galaxies forming a cosmic web at scales on which entire galaxies appear like tiny particles.<ref>{{cite web |title=Filaments of the Early Universe |url=https://www.science.org/content/article/filaments-early-universe |author=Schilling, Govert |date=23 May 2001|website=Science}}</ref><ref>{{cite web |title=The Cosmic Web of Galaxies, Dark Matter and How It Emerged |url=https://structures.uni-heidelberg.de/blog/posts/2022_12_cw/ |author=Stapelberg, Sebastian |date=5 December 2022|website=Structures Blog}}</ref>

In the standard [[Lambda-CDM model]] of cosmology, the [[mass–energy equivalence|mass–energy]] content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as [[dark energy]].<ref name="NASA Planck Mission">{{cite web |url=http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |title=Planck Mission Brings Universe into Sharp Focus |website=NASA Mission Pages |date=21 March 2013 |access-date=1 May 2016 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112001039/http://www.nasa.gov/mission_pages/planck/news/planck20130321.html }}</ref><ref name="NASA Science Dark Matter">{{cite web |url=https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ |title=Dark Energy, Dark Matter |website=NASA Science: Astrophysics |date=5 June 2015 |access-date=12 July 2017 |archive-date=2 June 2013 |archive-url=https://web.archive.org/web/20130602083852/http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ |url-status=live }}</ref><ref name="planck_overview">{{cite journal |first1=P. A. R. |last1=Ade |first2=N. |last2=Aghanim |author2-link=Nabila Aghanim |first3=C. |last3=Armitage-Caplan |collaboration=Planck Collaboration |title=Planck 2013 results. I. Overview of products and scientific results – Table&nbsp;9 |journal=[[Astronomy and Astrophysics]] |volume=1303 |page=5062 |url=http://www.cosmos.esa.int/web/planck/publications |date=22 March 2013 |arxiv=1303.5062 |bibcode=2014A&A...571A...1P |doi=10.1051/0004-6361/201321529 |s2cid=218716838 |display-authors=etal |archive-date=9 June 2016 |access-date=5 May 2016 |archive-url=https://web.archive.org/web/20160609180651/http://www.cosmos.esa.int/web/planck/publications |url-status=live }}</ref><ref name="wmap7parameters(a)">{{cite web |title=First Planck results: the Universe is still weird and interesting |url=https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/ |author=Francis, Matthew |date=22 March 2013 |website=Ars Technica |access-date=14 June 2017 |archive-date=2 May 2019 |archive-url=https://web.archive.org/web/20190502143413/https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/ |url-status=live }}</ref> Thus, dark matter constitutes 85% of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content.<ref name="planckcam">{{cite web |url=http://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |title=Planck captures portrait of the young Universe, revealing earliest light |date=21 March 2013 |publisher=University of Cambridge |access-date=21 March 2013 |archive-date=17 April 2019 |archive-url=https://web.archive.org/web/20190417165900/https://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |url-status=live }}</ref><ref name="DarkMatter">{{cite book |first=Sean |last=Carroll |year=2007 |publisher=The Teaching Company |title=Dark Matter, Dark Energy: The dark side of the universe |at=Guidebook Part&nbsp;2 p.&nbsp;46 <!-- access-date=7 October 2013 --> |quote=...&nbsp;dark matter: An invisible, essentially collisionless component of matter that makes up about 25&nbsp;percent of the energy density of the universe ... it's a different kind of particle... something not yet observed in the laboratory&nbsp;...}}</ref><ref>{{cite magazine |title=Dark matter |magazine=National Geographic Magazine |department=Hidden cosmos |url=http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text |archive-url=https://web.archive.org/web/20141225013843/http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text |archive-date=25 December 2014 |access-date=10 June 2015 |first=Timothy |last=Ferris |date=January 2015}}</ref><ref name="wmap7parameters">{{cite journal |last1=Jarosik |first1=N. |display-authors=etal |date=2011 |title=Seven-year Wilson microwave anisotropy probe (WMAP) observations: Sky maps, systematic errors, and basic results |journal=[[Astrophysical Journal Supplement]] |volume=192 |issue=2 |page=14 |arxiv=1001.4744 |bibcode=2011ApJS..192...14J |doi=10.1088/0067-0049/192/2/14|s2cid=46171526 }}</ref> While the density of dark matter is significant in the halo around a galaxy, its local density in the [[Solar System]] is much less than normal matter. The total of all the dark matter out to the orbit of Neptune would add up to about 10<sup>17</sup>&nbsp;kg, the same as a large asteroid.<ref>{{cite web |title=This Is How Much Dark Matter Passes Through Your Body Every Second |url=https://www.forbes.com/sites/startswithabang/2018/07/03/this-is-how-much-dark-matter-passes-through-your-body-every-second/ |author=Siegel, Ethan |date=3 July 2018 |website=Forbes |access-date=6 March 2025 |archive-date=20 December 2024 |archive-url=https://web.archive.org/web/20241220111123/https://www.forbes.com/sites/startswithabang/2018/07/03/this-is-how-much-dark-matter-passes-through-your-body-every-second/ |url-status=live }}</ref> Dark matter is classified as "cold", "warm", or "hot" according to [[velocity]] (more precisely, its [[free streaming]] length). Recent models have favored a [[cold dark matter]] scenario, in which [[Structure formation|structures emerge]] by the gradual accumulation of particles.

Dark matter is not known to interact with ordinary [[Baryonic matter#Baryonic matter|baryonic matter]] and [[radiation]] except through gravity, making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered [[subatomic particle]], such as either [[weakly interacting massive particle]]s (WIMPs) or [[axion]]s.<ref name="ars lensing">{{cite news |last1=Timmer |first1=John |date=21 April 2023 |title=No WIMPS! Heavy particles don't explain gravitational lensing oddities |language=en-us |work=Ars Technica |url=https://arstechnica.com/science/2023/04/gravitational-lensing-may-point-to-lighter-dark-matter-candidate/ |access-date=21 June 2023}}</ref> The other main possibility is that dark matter is composed of [[primordial black hole]]s.<ref name="Carr24">{{cite journal |last1=Carr |first1=B. J. |last2=Clesse |first2=S. |last3=García-Bellido |first3=J. |last4=Hawkins |first4=M. R. S. |last5=Kühnel |first5=F. |title=Observational evidence for primordial black holes: A positivist perspective |journal=Physics Reports |date=26 February 2024 |volume=1054 |pages=1–68 |doi=10.1016/j.physrep.2023.11.005 |url=https://www.sciencedirect.com/science/article/pii/S0370157323003976 |issn=0370-1573|arxiv=2306.03903 |bibcode=2024PhR..1054....1C }} See Figure 39.</ref><ref name="Bird">{{cite journal |last1=Bird |first1=Simeon |last2=Albert |first2=Andrea |last3=Dawson |first3=Will |last4=Ali-Haïmoud |first4=Yacine |last5=Coogan |first5=Adam |last6=Drlica-Wagner |first6=Alex |last7=Feng |first7=Qi |last8=Inman |first8=Derek |last9=Inomata |first9=Keisuke |last10=Kovetz |first10=Ely |last11=Kusenko |first11=Alexander |last12=Lehmann |first12=Benjamin V. |last13=Muñoz |first13=Julian B. |last14=Singh |first14=Rajeev |last15=Takhistov |first15=Volodymyr |last16=Tsai |first16=Yu-Dai |title=Primordial black hole dark matter |journal=Physics of the Dark Universe |date=1 August 2023 |volume=41 |article-number=101231 |doi=10.1016/j.dark.2023.101231 |arxiv=2203.08967 |bibcode=2023PDU....4101231B |s2cid=247518939 |issn=2212-6864}}</ref><ref name="Carr" />

Although the astrophysics community generally accepts the existence of dark matter,<ref>{{cite journal |url=https://www.scientificamerican.com/article/is-dark-matter-real/ |title=Is dark matter real? |date=August 2018 |last1=Hossenfelder |first1=Sabine |last2=McGaugh |first2=Stacy S. |journal=Scientific American |volume=319 |issue=2 |pages=36–43 |doi=10.1038/scientificamerican0818-36 |pmid=30020902 |bibcode=2018SciAm.319b..36H |s2cid=51697421 |quote=Right now a few dozens of scientists are studying modified gravity, whereas several thousand are looking for particle dark matter. |url-access=subscription |archive-date=28 September 2022 |access-date=27 December 2020 |archive-url=https://web.archive.org/web/20220928141536/https://www.scientificamerican.com/article/is-dark-matter-real/ |url-status=live }}</ref> a minority of astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include [[modified Newtonian dynamics]] (MOND), [[tensor–vector–scalar gravity]], and [[entropic gravity]]. So far, none of the proposed modified gravity theories can describe [[#Observational_evidence|every piece of observational evidence]] at the same time, suggesting that even if gravity has to be modified, some form of dark matter would still be required.<ref name="CarrollTrialogue" />

== History ==

=== 1884 to 1940 ===
The hypothesis of dark matter has an elaborate history.<ref name=GianfracoHooperHistory/><ref>{{cite journal |last1=de Swart |first1=J.G. |last2=Bertone |first2=G. |last3=van Dongen |first3=J. |year=2017 |title=How dark matter came to matter |journal=[[Nature Astronomy]] |volume=1 |issue=59 |page=59 |arxiv=1703.00013 |bibcode=2017NatAs...1E..59D |doi=10.1038/s41550-017-0059 |s2cid=119092226 }}</ref>
[[William Thomson, 1st Baron Kelvin|Lord Kelvin]] discussed the potential number of stars around the Sun in the appendices of a book based on a series of lectures given in 1884 in Baltimore.<ref name=Kelvin-1904/><ref name=GianfracoHooperHistory/> He inferred their density using the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20–100&nbsp;million years old. He posed what would happen if there were a thousand million stars within 1&nbsp;[[parsec|kiloparsec]] of the Sun (at which distance their parallax would be 1&nbsp;[[minute and second of arc|milli-arcsecond]]). Kelvin concluded:
<blockquote>"Many of our supposed thousand million stars &mdash; perhaps a great majority of them &mdash; may be dark bodies."<ref name=Kelvin-1904>{{cite book |last=Thompson |first=((W., Lord Kelvin)) |author-link=William Thomson, 1st Baron Kelvin |year=1904 |title=Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light |publisher=C.J. Clay and Sons |place=London, UK |page=274 |url=https://babel.hathitrust.org/cgi/pt?id=ien.35556038198842&view=1up&seq=304 |via=hathitrust.org }}</ref><ref name=ArsTech-2017-02-03>{{cite magazine |title=A history of dark matter |date=2017-02-03 |magazine=[[Ars Technica]] |url=https://arstechnica.com/science/2017/02/a-history-of-dark-matter/ |access-date=8 February 2017 |lang=en-us |archive-date=10 December 2019 |archive-url=https://web.archive.org/web/20191210132125/https://arstechnica.com/science/2017/02/a-history-of-dark-matter/ |url-status=live }}</ref></blockquote>

In 1906, [[Henri Poincaré]]<ref name=Poincaré-1906/> used the [[French language|French]] term [{{Lang|fr|matière obscure}}] ("dark matter") in discussing Kelvin's work.<ref name=Poincaré-1906>{{cite journal |last1=Poincaré |first1=H. |author-link=Henri Poincaré |year=1906 |title=La Voie lactée et la théorie des gaz |journal=Bulletin de la Société astronomique de France |volume=20 |pages=153–165 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112110949630&view=1up&seq=171 |trans-title=The Milky Way and the theory of gases |language=fr}}</ref><ref name=ArsTech-2017-02-03/> He concluded that the amount of dark matter would need to be less than that of visible matter, which was later found to be false.<ref name=ArsTech-2017-02-03/><ref name=GianfracoHooperHistory/>

The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer [[Jacobus Kapteyn]] in 1922.<ref>{{cite journal |first=J.C. |last=Kapteyn |author-link=Jacobus Kapteyn |year=1922 |title=First attempt at a theory of the arrangement and motion of the sidereal system |journal=Astrophysical Journal |volume=55 |pages=302–327 |bibcode=1922ApJ....55..302K |doi=10.1086/142670 |quote=It is incidentally suggested when the theory is perfected it may be possible to determine ''the amount of dark matter'' from its gravitational effect. {{grey|[''emphasis in original'']}} }}</ref><ref name=Patras2014>{{cite conference |last=Rosenberg |first=Leslie J. |date=30 June 2014 |title=Status of the Axion Dark-Matter Experiment (ADMX) |conference=10th PATRAS Workshop on Axions, WIMPs and WISPs |page=2 |url=http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |url-status=live |archive-url=https://web.archive.org/web/20160205163816/http://indico.cern.ch/event/300768/session/0/contribution/30/attachments/566901/780884/Rosenberg-Patras_30jun14.pdf |archive-date=2016-02-05 |conference-url=http://axion-wimp2014.desy.de }}</ref> A publication from 1930 by Swedish astronomer [[Knut Lundmark]] points to him being the first to hypothesize that the universe must contain much more mass than can be observed.<ref>{{cite journal |last=Lundmark |first=K. |author-link=Knut Lundmark |date=1930-01-01 |title=Über die Bestimmung der Entfernungen, Dimensionen, Massen, und Dichtigkeit fur die nächstgelegenen anagalacktischen Sternsysteme |lang=de |trans-title=On determination of distances, dimensions, masses, and densities for the nearest non-galactic star systems |journal=Meddelanden Fran Lunds Astronomiska Observatorium |volume=125 |pages=1–13 |bibcode=1930MeLuF.125....1L |url=https://ui.adsabs.harvard.edu/abs/1930MeLuF.125....1L }}</ref> Dutch [[radio astronomy]] pioneer [[Jan Oort]] also hypothesized the existence of dark matter in 1932.<ref name=Patras2014/><ref>{{cite journal |last=Oort |first=J.H. |author-link=Jan Oort |year=1932 |title=The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems |journal=Bulletin of the Astronomical Institutes of the Netherlands |volume=6 |pages=249–287 |bibcode=1932BAN.....6..249O }}</ref><ref>{{cite web |title=The hidden lives of galaxies: Hidden mass |website=Imagine the Universe |publisher=[[NASA]] / [[GSFC]] |place=Greenbelt, MD |url=http://imagine.gsfc.nasa.gov/teachers/galaxies/imagine/hidden_mass.html |access-date=5 May 2016 |archive-date=23 April 2015 |archive-url=https://web.archive.org/web/20150423103845/http://imagine.gsfc.nasa.gov/teachers/galaxies/imagine/hidden_mass.html |url-status=live }}</ref> Oort was studying stellar motions in [[Local Group|the galactic neighborhood]] and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be incorrect.<ref>{{cite journal |last1=Kuijken |first1=K. |last2=Gilmore |first2=G. |date=July 1989 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=239 |issue=2 |pages=651–664 |bibcode=1989MNRAS.239..651K |title=The Mass Distribution in the Galactic Disc – Part III – the Local Volume Mass Density |doi=10.1093/mnras/239.2.651 |doi-access=free }}</ref>

[[File:Hubble close-up on the Coma Cluster.jpg|thumb|Hubble close-up on the Coma Cluster<ref>{{cite news|title=Hubble close-up on the Coma Cluster|url=http://www.spacetelescope.org/images/potw1402a/|access-date=18 January 2014|newspaper=ESA/Hubble Picture of the Week}}</ref>]]

In 1933, Swiss astrophysicist [[Fritz Zwicky]] studied [[galaxy cluster]]s while working at [[California Institute of Technology|Caltech]] and made a similar inference.<ref name=zwicky1933>{{cite journal |last=Zwicky |first=F. |author-link=Fritz Zwicky |date=1933 |title=Die Rotverschiebung von extragalaktischen Nebeln |trans-title=The red shift of extragalactic nebulae |journal=[[Helvetica Physica Acta]] |volume=6 |pages=110–127 |bibcode=1933AcHPh...6..110Z }}</ref>{{efn
|''"Um, wie beobachtet, einen mittleren Dopplereffekt von 1000&nbsp;km/sek oder mehr zu erhalten, müsste also die mittlere Dichte im Comasystem mindestens 400&nbsp;mal grösser sein als die auf Grund von Beobachtungen an leuchtender Materie abgeleitete. Falls sich dies bewahrheiten sollte, würde sich also das überraschende Resultat ergeben, dass dunkle Materie in sehr viel grösserer Dichte vorhanden ist als leuchtende Materie."''<ref name=zwicky1933/>{{rp|style=ama|p=125}}
: [In order to obtain an average Doppler effect of 1000&nbsp;km/s or more, as observed, the average density in the Coma system would thus have to be at least 400&nbsp;times greater than that derived on the basis of observations of luminous matter. If this were to be confirmed, the surprising result would then follow that dark matter is present in very much greater density than luminous matter.]
}}<ref name="zwicky1937">{{cite journal |last=Zwicky |first=Fritz |author-link=Fritz Zwicky |date=1937 |title=On the Masses of Nebulae and of Clusters of Nebulae |journal=[[The Astrophysical Journal]] |volume=86 |pages=217–246 |bibcode=1937ApJ....86..217Z |doi=10.1086/143864 |doi-access=free}}</ref> Zwicky applied the [[virial theorem]] to the [[Coma Cluster]] and obtained evidence of unseen mass he called {{Lang|de|dunkle Materie}} ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated the cluster had about 400&nbsp;times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitational attraction to hold the cluster together.<ref>Some details of Zwicky's calculation and of more modern values are given in {{cite report |first=M. |last=Richmond |date=c. 1999 |title=Using the virial theorem: The mass of a cluster of galaxies |type=lecture notes |series=Physics&nbsp;440 |publisher=[[Rochester Institute of Technology]] |place=Rochester, NY |url=http://spiff.rit.edu/classes/phys440/lectures/gal_clus/gal_clus.html |via=spiff.rit.edu |access-date=10 July 2007 |archive-date=27 December 2014 |archive-url=https://web.archive.org/web/20141227062624/http://spiff.rit.edu/classes/phys440/lectures/gal_clus/gal_clus.html |url-status=live }}</ref> Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the [[Hubble constant]];<ref>{{cite book |first=Katherine |last=Freese |year=2014 |title=The Cosmic Cocktail: Three parts dark matter |publisher=Princeton University Press |isbn=978-1-4008-5007-5 |url={{google books |plainurl=y |id=c2B8AgAAQBAJ}} }}</ref> the same calculation today shows a smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that most of the gravitational matter present was dark.<ref name=ArsTech-2017-02-03/> However, unlike modern theories, Zwicky considered "dark matter" to be non-luminous ordinary matter.<ref name=GianfracoHooperHistory/>{{rp|III.A}}

Further indications of [[mass-to-light ratio]] anomalies came from measurements of [[galaxy rotation curve]]s. In 1939, [[Horace W. Babcock|H.W. Babcock]] reported the rotation curve for the [[Andromeda Galaxy]] (then called the Andromeda Nebula), which suggested the mass-to-luminosity ratio increases radially.<ref name=Babcock-1939>{{cite journal |last=Babcock |first=H.W. |author-link=Horace W. Babcock |year=1939 |title=The rotation of the Andromeda Nebula |journal=[[Lick Observatory Bulletin]] |volume=19 |pages=41–51 |bibcode=1939LicOB..19...41B |doi=10.5479/ADS/bib/1939LicOB.19.41B |doi-access=free }}</ref> He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral, rather than to unseen matter. Following [[Horace W. Babcock|Babcock's]] 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda Galaxy and a mass-to-light ratio of 50; in 1940, [[Jan Oort|Oort]] discovered and wrote about the large non-visible halo of [[NGC 3115|NGC&nbsp;3115]].<ref>{{cite journal |last=Oort |first=J.H. |author-link=Jan Oort |date=April 1940 |title=Some problems concerning the structure and dynamics of the galactic system and the elliptical nebulae NGC&nbsp;3115 and 4494 |journal=[[The Astrophysical Journal]] |volume=91 |issue=3 |pages=273–306 |bibcode=1940ApJ....91..273O |doi=10.1086/144167 |hdl=1887/8533 |hdl-access=free |url=https://openaccess.leidenuniv.nl/bitstream/handle/1887/8533/008_653_032.pdf?sequence=1 |via=leidenuniv.nl |archive-date=29 July 2020 |access-date=4 September 2019 |archive-url=https://web.archive.org/web/20200729023303/https://openaccess.leidenuniv.nl/bitstream/handle/1887/8533/008_653_032.pdf?sequence=1 |url-status=live }}</ref>

=== 1970s ===
The hypothesis of dark matter largely took root in the 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in [[Princeton, New Jersey]], by [[Jeremiah Ostriker]], [[Jim Peebles]], and {{ill|Amos Yahil|qid=Q124254926|s=1}}, and in Tartu, Estonia, by [[Jaan Einasto]], {{ill|Enn Saar|et}}, and {{ill|Ants Kaasik|et}}.<ref name=DeSwart2024>{{cite journal |last1=de Swart |first1=Jaco |date=1 August 2024 |title=Five decades of missing mass |journal=[[Physics Today]] |volume=77 |pages=34–43 | doi=10.1063/pt.ozhk.lfeb |doi-access=free }}</ref>

[[File:Galaxy rotation under the influence of dark matter.ogv|thumb|Left: A simulated galaxy without dark matter. Right: Galaxy with a flat rotation curve that would be expected with dark matter.]]

One of the observations that served as evidence for the existence of galactic halos of dark matter was the shape of [[galaxy rotation curve]]s. These observations were done in optical and radio astronomy. In optical astronomy, [[Vera Rubin]] and [[Kent Ford (astronomer)|Kent Ford]] worked with a new [[spectrograph]] to measure the [[galaxy rotation curve|velocity curve]] of edge-on [[spiral galaxy|spiral galaxies]] with greater accuracy.<ref name=NYT-20161227>{{cite news |last=Overbye |first=D. |author-link=Dennis Overbye |date=27 December 2016 |title=Vera Rubin, 88, dies; opened doors in astronomy, and for women |type=obituary |newspaper=[[The New York Times]] |url=https://www.nytimes.com/2016/12/27/science/vera-rubin-astronomist-who-made-the-case-for-dark-matter-dies-at-88.html |access-date=27 December 2016 |archive-date=26 September 2019 |archive-url=https://web.archive.org/web/20190926002332/https://www.nytimes.com/2016/12/27/science/vera-rubin-astronomist-who-made-the-case-for-dark-matter-dies-at-88.html |url-status=live }}</ref><ref>{{cite web |title=First observational evidence of dark matter |website=Darkmatterphysics.com |url=http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |access-date=6 August 2013 |archive-url=https://web.archive.org/web/20130625183052/http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm |archive-date=25 June 2013}}</ref><ref name=Rubin1970>{{cite journal |last1=Rubin |first1=V.C. |author1-link=Vera Rubin |last2=Ford |first2=W.K. Jr. |author2-link=Kent Ford (astronomer) |date=February 1970 |title=Rotation of the Andromeda nebula from a spectroscopic survey of emission regions |journal=[[The Astrophysical Journal]] |volume=159 |pages=379–403 |bibcode=1970ApJ...159..379R |doi=10.1086/150317 |s2cid=122756867 }}</ref>

At the same time, radio astronomers were making use of new [[radio telescope]]s to map the [[21 cm line|21&nbsp;cm line]] of [[atomic hydrogen]] in nearby galaxies. The radial distribution of interstellar atomic hydrogen ([[H I region|H{{sup|{{math|I}}}}]]) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of the [[Andromeda Galaxy]] with the {{convert|300|ft|m|adj=mid}} telescope at [[Green Bank Observatory|Green Bank]]<ref name=Roberts1966>{{cite journal |last1=Roberts |first1=Morton S. <!-- |author-link1=Morton S. Roberts (astronomer) --> |date=May 1966 |title=A high-resolution 21&nbsp;cm hydrogen-line survey of the Andromeda nebula |journal=[[The Astrophysical Journal]] |volume=159 |pages=639–656 |bibcode=1966ApJ...144..639R |doi=10.1086/148645}}</ref> and the {{convert|250|ft|m|adj=mid}} dish at [[Jodrell Bank Observatory|Jodrell Bank]]<ref name="Gottesman1966">{{cite journal |last1=Gottesman |first1=S. T. <!-- |author-link1=S. T. Gottesman (astronomer) --> |last2=Davies |first2=Rod D. |author-link2=Rod Davies |last3=Reddish |first3=Vincent Cartledge |author-link3=Vincent Cartledge Reddish |date=1966 |title=A neutral hydrogen survey of the southern regions of the Andromeda nebula |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=133 |issue=4 |pages=359–387 |bibcode=1966MNRAS.133..359G |doi=10.1093/mnras/133.4.359 |doi-access=free}}</ref> already showed the H{{sup|{{math|I}}}} rotation curve did not trace the decline expected from Keplerian orbits.

As more sensitive receivers became available, <!-- [[Morton S. Roberts|-->Roberts<!--]]--> & <!-- [[Robert N. Whitehurst (astronomer) --- not the Olympic sailor --- |-->Whitehurst<!--]]--> (1975)<ref name=Roberts1975>{{cite journal |last1=Roberts |first1=Morton S. <!-- |author-link1=Morton S. Roberts |last2=Whitehurst |first2=Robert N.  |author-link2=Robert N. Whitehurst (astronomer) --> |date=October 1975 |title=The rotation curve and geometry of M&nbsp;31 at large galactocentric distances |journal=[[The Astrophysical Journal]] |volume=201 |pages=327–346 |bibcode=1975ApJ...201..327R |doi=10.1086/153889}}</ref> were able to trace the rotational velocity of Andromeda to 30&nbsp;kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii; that paper's ''Figure&nbsp;16''<ref name=Roberts1975/> combines the optical data<ref name=Rubin1970/> (the cluster of points at radii of less than 15&nbsp;kpc with a single point further out) with the H{{sup|{{math|I}}}} data between 20 and 30&nbsp;kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic H{{sup|{{math|I}}}} spectroscopy was being developed. <!-- [[David H. Rogstad (astronomer)| -->Rogstad<!-- ]] --> & [[Seth Shostak|Shostak]] (1972)<ref name=Rogstad1972/> published H{{sup|{{math|I}}}} rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended H{{sup|{{math|I}}}}&nbsp;disks.<ref name="Rogstad1972">{{cite journal |last1=Rogstad |first1=David H. <!-- |author-link1=David H. Rogstad (astronomer) --> |last2=Shostak |first2=G. Seth |author-link2=Seth Shostak |date=September 1972 |title=Gross properties of five Scd galaxies as determined from 21&nbsp;centimeter observations |journal=[[The Astrophysical Journal]] |volume=176 |pages=315–321 |bibcode=1972ApJ...176..315R |doi=10.1086/151636}}</ref> In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the [[Westerbork Synthesis Radio Telescope]].<ref>{{cite thesis |last=Bosma |first=A. |date=1978 |title=The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types |degree=Ph.D. |publisher=[[Rijksuniversiteit Groningen]] |url=http://nedwww.ipac.caltech.edu/level5/March05/Bosma/frames.html |archive-date=14 May 2011 |access-date=3 February 2012 |archive-url=https://web.archive.org/web/20110514103648/http://nedwww.ipac.caltech.edu/level5/March05/Bosma/frames.html |url-status=live }}</ref>

In 1978, [[Gary Steigman|Steigman]] et al.<ref>{{cite journal |last1=Gunn |first1=J. E. |last2=Lee |first2=B. W. |last3=Lerche |first3=I. |last4=Schramm |first4=D. N. |last5=Steigman |first5=G. |date=Aug 1978 |title=Some astrophysical consequences of the existence of a heavy stable neutral lepton. |url=https://ui.adsabs.harvard.edu/abs/1978ApJ...223.1015G/abstract |journal=The Astrophysical Journal |language=en |volume=223 |pages=1015–1031 |doi=10.1086/156335 |bibcode=1978ApJ...223.1015G |issn=0004-637X |archive-date=29 April 2024 |access-date=26 April 2025 |archive-url=https://web.archive.org/web/20240429134658/https://ui.adsabs.harvard.edu/abs/1978ApJ...223.1015G/abstract |url-status=live }}</ref> presented a study that extended earlier cosmological [[Relic abundance|relic particle density]] calculations to any hypothetical stable, electrically neutral, weak-scale lepton, showing how such a particle's abundance would "freeze out" in the [[Early universe|early Universe]] and providing analytic expressions that linked its mass and weak interaction cross-section to the present-day matter density. By decoupling the analysis from specific [[neutrino]] properties and treating the candidate generically, the authors set out a framework that later became the standard template for [[weakly interacting massive particle]]s (WIMPs)<ref>{{cite book |last1=Tan |first1=Chung-i |url=https://books.google.com/books?id=CtBKDwAAQBAJ |title=Particles, Strings And Supernovae - Proceedings Of Theoretical Advanced Study Institute In Elementary Particle Physics (In 2 Volumes) |last2=Jevicki |first2=Antal |date=1989-05-01 |publisher=World Scientific |isbn=978-981-4590-77-8 |page=191 |language=en}}</ref> and for comparing [[Particle physics|particle-physics]] models with cosmological constraints. Though subsequent work has refined the methodology and explored many alternative candidates, this paper marked the first explicit, systematic treatment of dark matter as a new particle species beyond the [[Standard Model]].<ref>{{citation |last=Mambrini |first=Yann |title=Introduction |date=2021 |work=Particles in the Dark Universe: A Student's Guide to Particle Physics and Cosmology |pages=1–22 |editor-last=Mambrini |editor-first=Yann |url=https://link.springer.com/chapter/10.1007/978-3-030-78139-2_1 |access-date=2025-04-26 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-78139-2_1 |isbn=978-3-030-78139-2|url-access=subscription }}</ref> By the late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy.<ref name=DeSwart2024/>

=== 1980s and 90s ===
[[Image:Gravitational lens-full.jpg|thumb|Gravitational lensing bends light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.]]

A stream of observations in the 1980–1990s supported the presence of dark matter. {{harvp|Persic|Salucci|Stel|1996}} is notable for the investigation of 967&nbsp;spirals.<ref>{{cite journal |first1=Massimo |last1=Persic |first2=Paolo |last2=Salucci |first3=Fulvio |last3=Stel |year=1996 |title=The universal rotation curve of spiral galaxies — I.&nbsp;The dark matter connection |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=281 |issue=1 |pages=27–47 |doi= 10.1093/mnras/278.1.27 |doi-access=free |arxiv=astro-ph/9506004 |bibcode= 1996MNRAS.281...27P }}</ref> The evidence for dark matter also included [[gravitational lensing]] of background objects by [[galaxy cluster]]s,<ref name=Randall_2015/>{{rp|style=ama|pp= 14–16}} the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the [[cosmic microwave background]].

=== 2000s to present ===

Since the turn of the millennium, the search for particle dark matter has been dominated by the hypothesis of [[weakly interacting massive particles]] (WIMPs), driven by hypothesized connections to [[supersymmetry]]. Experimental efforts were characterized by a rapid increase in sensitivity using liquid [[xenon]] detectors, including [[XENON]], [[Large Underground Xenon experiment|LUX]], [[PandaX]], and [[LZ experiment|LUX-ZEPLIN]] (LZ). Despite pushing interaction limits down by orders of magnitude, these direct detection experiments all reported null results for WIMPs across the standard GeV–TeV mass range.<ref name="Akerib2017">{{cite journal |last=Akerib |first=D. S. |display-authors=et al. |title=Results from a Search for Dark Matter in the Complete LUX Exposure |journal=[[Physical Review Letters]] |volume=118 |issue=2 |article-number=021303 |date=2017 |doi=10.1103/PhysRevLett.118.021303 |pmid=28128598 |arxiv=1608.07648 |bibcode=2017PhRvL.118b1303A }}</ref><ref name="Aprile2018">{{cite journal |last=Aprile |first=E. |display-authors=et al. |collaboration=XENON Collaboration |title=Dark Matter Search Results from a One Ton-Year Exposure of XENON1T |journal=[[Physical Review Letters]] |volume=121 |issue=11 |article-number=111302 |date=2018 |doi=10.1103/PhysRevLett.121.111302 |pmid=30265108 |arxiv=1805.12562 |bibcode=2018PhRvL.121k1302A }}</ref> As of late 2025, the LZ experiment had excluded WIMP cross-sections above 9 GeV/c<sup>2</sup> and reported the first detection of [[boron-8]] solar [[neutrino]]s via [[coherent elastic neutrino-nucleus scattering]] in a dark matter detector; this marks the experimental entry into the neutrino floor "fog," an irreducible background of neutrino noise that complicates future WIMP searches.<ref name="LZ2025">{{cite web |title=LZ dark matter experiment sets a world's best and spots neutrinos from the sun's core |url=https://www.llnl.gov/article/53711/lz-dark-matter-experiment-sets-worlds-best-spots-neutrinos-suns-core |publisher=[[Lawrence Livermore National Laboratory]] |date=8 December 2025 |access-date=14 January 2026 |archive-date=22 January 2026 |archive-url=https://web.archive.org/web/20260122103937/https://www.llnl.gov/article/53711/lz-dark-matter-experiment-sets-worlds-best-spots-neutrinos-suns-core |url-status=live }}</ref> Concurrently, the failure of the [[Large Hadron Collider]] to detect supersymmetric particles has constrained the theoretical parameter space for WIMPs.<ref>{{cite journal |last=Canepa |first=Anadi |title=Searches for Supersymmetry at the Large Hadron Collider |journal=Reviews in Physics |volume=4 |article-number=100033 |year=2019 |doi=10.1016/j.revip.2019.100033 |doi-access=free |bibcode=2019RvPhy...400033C }}</ref> These constraints have shifted significant focus toward alternative candidates such as [[axion]]s. The [[Axion Dark Matter Experiment]] achieved sensitivity to the plausible DFSZ axion model in the micro-electronvolt range by the early 2020s.<ref name="Braine2020">{{cite journal |last=Braine |first=T. |display-authors=et al. |collaboration=ADMX Collaboration |title=Extended Search for the Invisible Axion with the Axion Dark Matter Experiment |journal=[[Physical Review Letters]] |volume=124 |issue=10 |article-number=101303 |date=2020 |doi=10.1103/PhysRevLett.124.101303 |pmid=32216421 |arxiv=1910.08638 |bibcode=2020PhRvL.124j1303B }}</ref><ref>{{cite journal |last=Rybka |first=Gray |collaboration=ADMX Collaboration |title=Search for Axion Dark Matter from 1.1 to 1.3 GHZ with ADMX |journal=Physical Review Letters |arxiv=2504.07279 |date=9 April 2025 |volume=135 |issue=19 |article-number=191001 |doi=10.1103/d7mg-6sqq |pmid=41269976 |bibcode=2025PhRvL.135s1001C }}</ref>

The prevailing view among cosmologists remains that dark matter is composed primarily of some type of not-yet-characterized [[subatomic particle]].<ref name="Copi 1995">{{cite journal |last1=Copi |first1=C.J. |last2=Schramm |first2=D.N. |last3=Turner |first3=M.S. |year=1995 |title=Big-Bang nucleosynthesis and the baryon density of the universe |journal=[[Science (journal)|Science]] |volume=267 |issue=5195 |pages=192–199 |arxiv=astro-ph/9407006 |bibcode=1995Sci...267..192C |doi=10.1126/science.7809624 |pmid=7809624 |s2cid=15613185 |url=https://cds.cern.ch/record/265576 |archive-date=14 August 2019 |access-date=21 June 2023 |archive-url=https://web.archive.org/web/20190814070006/https://cds.cern.ch/record/265576 |url-status=live }}</ref><ref name="Bergstrom 2000">{{cite journal |last=Bergstrom |first=L. |year=2000 |title=Non-baryonic dark matter: Observational evidence and detection methods |journal=[[Reports on Progress in Physics]] |volume=63 |issue=5 |pages=793–841 |arxiv=hep-ph/0002126 |bibcode=2000RPPh...63..793B |doi=10.1088/0034-4885/63/5/2r3 |s2cid=119349858 }}</ref> While this remains the majority opinion, the lack of particle detection has led to a divergence in consensus, with macroscopic candidates such as [[primordial black holes]] seeing renewed interest following observations by [[LIGO]] and the [[James Webb Space Telescope]].<ref name="Carr24" /><ref>{{cite journal |last1=Cooley |first1=Jodi |last2=Dutta |first2=Bhaskar |last3=Yu |first3=Hai-Bo |title=Dark matter candidates and searches |journal=[[Canadian Journal of Physics]] |date=April 2024 |volume=102 |issue=4 |pages=231–252 |doi=10.1139/cjp-2024-0128 |arxiv=2410.23454 |bibcode=2024CaJPh.103.0128B }}</ref> The search for such particles, by a variety of means, is one of the major efforts in [[particle physics]].<ref name="bertone hooper silk">{{cite journal |last1=Bertone |first1=G. |last2=Hooper |first2=D. |last3=Silk |first3=J. |year=2005 |title=Particle dark matter: Evidence, candidates, and constraints |journal=[[Physics Reports]] |volume=405 |issue=5–6 |pages=279–390 |arxiv=hep-ph/0404175 |bibcode=2005PhR...405..279B |doi=10.1016/j.physrep.2004.08.031 |s2cid=118979310 }}</ref>

== Technical definition ==
{{see also|Friedmann equations}}
In standard cosmological calculations, ''"matter"'' means any constituent of the universe whose energy density scales with the inverse cube of the [[scale factor (cosmology)|scale factor]], i.e., {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−3}} }}.}} This is in contrast to ''"radiation"'', which scales as the inverse fourth power of the scale factor {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−4}} }},}} and a [[cosmological constant]], which does not change with respect to {{mvar|a}} ({{nobr|{{math|''ρ'' ∝ ''a''{{sup|0}}}}}}).<ref name="Baumann lecture notes" /> The different scaling factors for matter and radiation are a consequence of radiation [[redshift]]. For example, after doubling the diameter of the observable Universe via [[cosmic expansion]], the scale, {{mvar|a}}, has doubled. The energy of the [[cosmic microwave background radiation]] has been halved (because the wavelength of each photon has doubled);<ref>{{cite news |last1=Siegel |first1=Ethan |title=Is energy conserved when photons redshift in our expanding universe? |year=2019 |work=[[Starts With a Bang]] |url=https://www.forbes.com/sites/startswithabang/2019/08/14/is-energy-conserved-when-photons-redshift-due-to-the-expanding-universe/?sh=745b3e3a3efa |access-date=5 November 2022 |language=en}}</ref> the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.{{efn
|However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.
}} The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.<ref name="Baumann lecture notes">{{cite web |first=Daniel |last=Baumann |title=Cosmology: Part&nbsp;III |department=Mathematical Tripos |publisher=Cambridge University |pages=21–22 |url=http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |access-date=24 January 2017 |archive-url=https://web.archive.org/web/20170202065045/http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf |archive-date=2 February 2017 }}</ref>

In principle, "dark matter" means all components of the universe which are not visible but still obey {{nobr|{{math|''ρ'' ∝ ''a''{{sup|−3}} }}.}} In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "[[missing baryon problem|missing baryons]]".<ref>{{cite arXiv |author=Peter |first=Annika H. G. |date=18 Jan 2012 |title=Dark Matter: A Brief Review |class=astro-ph.CO |eprint=1201.3942 }}</ref> Context will usually indicate which meaning is intended.

== Observational evidence ==
=== Galaxy rotation curves ===
[[File:Rotation_Curve_UGC11455.svg|thumb|upright=1.8|The rotation curve of spiral galaxy UCG11455.<ref name="w699">{{cite journal | last1=Mistele | first1=Tobias | last2=McGaugh | first2=Stacy | last3=Lelli | first3=Federico | last4=Schombert | first4=James | last5=Li | first5=Pengfei | title=Indefinitely Flat Circular Velocities and the Baryonic Tully–Fisher Relation from Weak Lensing | journal=The Astrophysical Journal Letters | volume=969 | issue=1 | date=2024-07-01 | issn=2041-8205 | doi=10.3847/2041-8213/ad54b0 | doi-access=free | page=L3 | arxiv=2406.09685 | bibcode=2024ApJ...969L...3M }}</ref><ref name="u758">{{cite journal | last1=Lelli | first1=Federico | last2=McGaugh | first2=Stacy S. | last3=Schombert | first3=James M. | title=Sparc: Mass Models for 175 Disk Galaxies with Spitzer Photometry and Accurate Rotation Curves | journal=The Astronomical Journal | volume=152 | issue=6 | date=2016-12-01 | issn=0004-6256 | doi=10.3847/0004-6256/152/6/157 | doi-access=free | page=157 | arxiv=1606.09251 | bibcode=2016AJ....152..157L }}</ref> The observed rotation for spiral galaxy UCG11455 is shown as points. The expected rotation from normal matter is shown in the line below.]]
{{Main|Galaxy rotation curve}}
The arms of [[spiral galaxies]] rotate around their galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then the galaxy can be modelled as a point mass in the centre and test masses orbiting around it, similar to the [[Solar System]].<ref group=lower-alpha>This is a consequence of the [[shell theorem]] and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).</ref> From [[Kepler's Third Law]], it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.<ref>{{cite journal |last=Salucci |first=P. |title=The distribution of dark matter in galaxies |journal=[[The Astronomy and Astrophysics Review]] |date=2019 |volume= 27 |issue= 1 |article-number= 2 |doi=10.1007/s00159-018-0113-1 |arxiv= 1811.08843 |bibcode= 2019A&ARv..27....2S}}</ref> Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases.

If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there may be a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

=== Velocity dispersions ===
{{Main|Velocity dispersion}}
Stars in bound systems must obey the [[virial theorem]]. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies<ref>{{cite journal |last1=Faber |first1=S. M. |last2=Jackson |first2=R. E. |date=1976 |title=Velocity dispersions and mass-to-light ratios for elliptical galaxies |journal=The Astrophysical Journal |volume=204 |pages=668–683 |bibcode=1976ApJ...204..668F |doi=10.1086/154215}}</ref> do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.<ref>{{cite book |last1=Binny |first1=James |last2=Merrifield |first2=Michael |year=1998 |title=Galactic Astronomy |publisher=Princeton University Press |pages=712–713}}</ref> As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.

=== Galaxy clustering ===
[[Galaxy clusters]] are particularly important for dark matter studies since their masses can be estimated in three independent ways:
* From the scatter in radial velocities of the galaxies within clusters
* From [[X-ray]]s emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
* [[Gravitational lens]]ing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.<ref>{{cite journal |title=Cosmological Parameters from Clusters of Galaxies |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=49 |issue=1 |pages=409–470 |doi=10.1146/annurev-astro-081710-102514 |arxiv=1103.4829 |bibcode=2011ARA&A..49..409A |year=2011 |last1=Allen |first1=Steven W. |last2=Evrard |first2=August E. |last3=Mantz |first3=Adam B. |s2cid=54922695 }}</ref>

[[File:A slice through the Universe.webm|thumb|The positions in space of the galaxies identified by the VIPERS survey.]]

On larger scales, large galaxy [[redshift survey]]s may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed [[redshift]]s; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the [[2dF Galaxy Redshift Survey]].<ref>{{cite journal |last1=Peacock |first1=J. |display-authors=etal |title=A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey |journal=Nature |year=2001 |volume=410 |issue=6825 |pages=169–173 |arxiv=astro-ph/0103143 |bibcode=2001Natur.410..169P |doi=10.1038/35065528 |pmid=11242069|s2cid=1546652 }}</ref> Results are in agreement with the [[Lambda-CDM model]].

=== Bullet Cluster ===
[[File:Bullet cluster.jpg|thumb|The Bullet Cluster]]
{{Main|Bullet Cluster}}
The bullet cluster is the result of a recent collision of two galaxy clusters. It is of particular note because the location of the [[center of mass]] as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain.<ref>{{cite conference |url=http://cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-url=https://web.archive.org/web/20060821074820/http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf |archive-date=2006-08-21 |url-status=live |title=Dark matter and the Bullet Cluster |author1=Markevitch, M. |author2=Randall, S. |author3=Clowe, D. |author4=Gonzalez, A. |author5=Bradac, M. |name-list-style=amp |conference=36th COSPAR Scientific Assembly |date=16–23 July 2006 |location=Beijing, China}} Abstract only</ref><ref>{{cite journal |last1=Clowe |first1=Douglas |display-authors=etal |year=2006 |title=A Direct Empirical Proof of the Existence of Dark Matter |journal=The Astrophysical Journal Letters |volume=648 |issue=2 |pages=L109–L113 |arxiv=astro-ph/0608407 |doi=10.1086/508162 |bibcode=2006ApJ...648L.109C|s2cid=2897407 }}</ref><ref>{{cite web |url=https://arstechnica.com/science/2017/09/science-in-progress-did-the-bullet-cluster-withstand-scrutiny/ |title=Science-in-progress: Did the Bullet Cluster withstand scrutiny? |website=Ars Technica |author=Lee, Chris |date=21 September 2017 |access-date=17 April 2018 |archive-date=18 April 2018 |archive-url=https://web.archive.org/web/20180418092854/https://arstechnica.com/science/2017/09/science-in-progress-did-the-bullet-cluster-withstand-scrutiny/ |url-status=live }}</ref><ref>{{cite magazine |url=https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#3032b6081738 |title=The Bullet Cluster proves dark matter exists, but not for the reason most physicists think |magazine=Forbes |author=Siegel, Ethan |author-link=Ethan Siegel |date=9 November 2017}}</ref> Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to the dark matter separating from the visible gas, producing the separate lensing peak as observed.<ref>{{cite web|url=https://chandra.harvard.edu/graphics/resources/handouts/lithos/bullet_lithos.pdf|title=Bullet Cluster: Direct Proof of Dark Matter|publisher=NASA}}</ref>

=== Gravitational lensing ===
One of the consequences of [[general relativity]] is the [[gravitational lens]]. Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a [[galaxy cluster|cluster of galaxies]] lying between a more distant source such as a [[quasar]] and an observer. In this case, the galaxy cluster will lens the quasar.

Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including [[Abell 1689]].<ref>{{cite journal |last1=Taylor |first1=A. N. |display-authors=etal |date=1998 |title=Gravitational lens magnification and the mass of Abell&nbsp;1689 |journal=The Astrophysical Journal |volume=501 |issue=2 |pages=539–553 |arxiv=astro-ph/9801158 |bibcode=1998ApJ...501..539T |doi=10.1086/305827 |s2cid=14446661}}</ref> By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the [[weak gravitational lensing|weak]] regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.<ref>{{cite journal |last1=Refregier |first1=A. |year=2003 |title=Weak gravitational lensing by large-scale structure |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=41 |issue=1 |pages=645–668 |arxiv=astro-ph/0307212 |bibcode=2003ARA&A..41..645R |doi=10.1146/annurev.astro.41.111302.102207|s2cid=34450722 }}</ref><ref>{{cite journal |last1=Wu |first1=X. |last2=Chiueh |first2=T. |last3=Fang |first3=L. |last4=Xue |first4=Y. |date=1998 |title=A comparison of different cluster mass estimates: consistency or discrepancy? |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=301 |issue=3 |pages=861–871 |arxiv=astro-ph/9808179 |bibcode=1998MNRAS.301..861W |doi=10.1046/j.1365-8711.1998.02055.x |doi-access=free |citeseerx=10.1.1.256.8523|s2cid=1291475 }}</ref>

=== Type Ia supernova distance measurements ===
[[File:SNIacurva.png|thumb|Type Ia supernova luminosity relative to the Sun ([[Solar luminosity|''L''<sub>0</sub>]]) versus time shows the characteristic light curve for a Type Ia supernova. The peak is primarily due to the decay of [[nickel]] (Ni), while the later stage is powered by [[cobalt]] (Co).]]
{{Main|Type Ia supernova|Shape of the universe}}
Type Ia [[supernovae]] can be used as [[standard candles]] to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past.<ref>{{cite journal |last1=Planck Collaboration |last2=Aghanim |first2=N.|author2-link=Nabila Aghanim |last3=Akrami |first3=Y. |last4=Ashdown |first4=M. |last5=Aumont |first5=J. |last6=Baccigalupi |first6=C. |last7=Ballardini |first7=M. |last8=Banday |first8=A. J. |last9=Barreiro |first9=R. B. |last10=Bartolo |first10=N. |last11=Basak |first11=S. |title=Planck 2018 results. VI. Cosmological parameters |journal=Astronomy & Astrophysics |year=2020 |volume=641 |pages=A6 |doi=10.1051/0004-6361/201833910 |arxiv=1807.06209 |bibcode=2020A&A...641A...6P |s2cid=119335614}}</ref> Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to [[dark energy]].<ref>{{cite journal |last1=Kowalski |first1=M. |display-authors=etal |year=2008 |title=Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets |journal=The Astrophysical Journal |volume=686 |issue=2 |pages=749–778 |arxiv=0804.4142 |bibcode=2008ApJ...686..749K |doi=10.1086/589937 |s2cid=119197696 }}</ref> Since observations indicate the universe is almost flat,<ref name="NASA_Shape">{{cite web |url=https://map.gsfc.nasa.gov/universe/uni_shape.html |title=Will the Universe expand forever? |publisher=NASA |date=24 January 2014 |access-date=2021-03-28 |archive-date=31 March 2019 |archive-url=https://web.archive.org/web/20190331105235/https://map.gsfc.nasa.gov/universe/uni_shape.html |url-status=live }}</ref><ref name="Fermi_Flat">{{cite web|url= https://www.symmetrymagazine.org/article/april-2015/our-flat-universe |title=Our flat universe |publisher=FermiLab/SLAC |date=7 April 2015 |access-date=2021-03-28}}</ref><ref>{{cite journal |title=Unexpected connections |first=Marcus Y. |last=Yoo |journal=Engineering & Science |volume=74 |issue=1 |date=2011 |page=30}}</ref> it is expected the total energy density of everything in the universe should sum to 1 ({{nowrap|Ω<sub>tot</sub> ≈ 1}}). The measured dark energy density is {{nowrap|Ω<sub>Λ</sub> ≈ 0.690}}; the observed ordinary (baryonic) matter energy density is {{nowrap|Ω<sub>b</sub> ≈ 0.0482}} and the energy density of radiation is negligible. This leaves a missing {{nowrap|Ω<sub>dm</sub> ≈ 0.258}} which nonetheless behaves like matter (see technical definition section above){{snd}}dark matter.<ref name="planckesa2015">{{cite web |url=http://www.cosmos.esa.int/web/planck/publications |title=Planck Publications: Planck 2015 Results |publisher=European Space Agency |date=February 2015 |access-date=9 February 2015 |archive-date=9 June 2016 |archive-url=https://web.archive.org/web/20160609180651/http://www.cosmos.esa.int/web/planck/publications |url-status=live }}</ref>

=== Lyman-alpha forest ===
{{Main|Lyman-alpha forest}}
In [[astronomical spectroscopy]], the Lyman-alpha forest is the sum of the [[spectral line|absorption lines]] arising from the [[Lyman series|Lyman-alpha]] transition of [[hydrogen line|neutral hydrogen]] in the spectra of distant [[galaxy|galaxies]] and [[quasar]]s. Lyman-alpha forest observations can also constrain cosmological models.<ref>{{cite journal |last1=Viel |first1=M. |last2=Bolton |first2=J. S. |last3=Haehnelt |first3=M. G. |date=2009 |title=Cosmological and astrophysical constraints from the Lyman α forest flux probability distribution function |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=399 |issue=1 |pages=L39–L43 |arxiv=0907.2927 |bibcode=2009MNRAS.399L..39V |doi=10.1111/j.1745-3933.2009.00720.x |s2cid=12470622 |doi-access=free}}</ref> These constraints agree with those obtained from WMAP data.

=== Cosmic microwave background ===
{{Main|Cosmic microwave background}}
Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via [[Thomson scattering]]. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB.

The CMB is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters.<ref name="Wayne Hu">The details are technical. For an intermediate-level introduction, see {{cite web |author=Hu, Wayne |title=Intermediate Guide to the Acoustic Peaks and Polarization |url=http://background.uchicago.edu/~whu/intermediate/intermediate.html |year=2001}}</ref>

The CMB anisotropy was first discovered by [[Cosmic Background Explorer|COBE]] in 1992, though this had too coarse resolution to detect the acoustic peaks.
After the discovery of the first acoustic peak by the balloon-borne [[BOOMERanG]] experiment in 2000, the power spectrum was precisely observed by [[WMAP]] in 2003–2012, and even more precisely by the [[Planck spacecraft|''Planck'' spacecraft]] in 2013–2015. The results support the Lambda-CDM model.<ref name="Hinshaw2009">{{cite journal |last1=Hinshaw |first1=G. |display-authors=etal |year=2009 |title=Five-year Wilkinson microwave anisotropy probe (WMAP) observations: Data processing, sky maps, and basic results |journal=The Astrophysical Journal Supplement |volume=180 |issue=2 |pages=225–245 |arxiv=0803.0732 |bibcode=2009ApJS..180..225H |doi=10.1088/0067-0049/180/2/225|s2cid=3629998 }}</ref><ref name="Planck15" /> The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the [[Lambda-CDM model]],<ref name="Planck15">{{cite journal |last1=Ade |first1=P.A.R. |display-authors=etal |title=Planck 2015 results. XIII. Cosmological parameters |journal=Astron. Astrophys. |year=2016 |volume=594 |issue=13 |page=A13 |doi=10.1051/0004-6361/201525830 |bibcode=2016A&A...594A..13P |arxiv=1502.01589 |s2cid=119262962 }}</ref> but difficult to reproduce with any competing model such as [[modified Newtonian dynamics]] (MOND).<ref>{{cite journal |last1=Skordis |first1=C. |display-authors=etal |title=Large scale structure in Bekenstein's theory of relativistic modified Newtonian dynamics |journal=Phys. Rev. Lett. |year=2006 |volume=96 |issue=1 |article-number=011301 |doi=10.1103/PhysRevLett.96.011301 |pmid=16486433 |bibcode=2006PhRvL..96a1301S |arxiv=astro-ph/0505519|s2cid=46508316 }}</ref>

=== Structure formation ===
{{Main|Structure formation}}
[[File:Dark matter map of KiDS survey region (region G12).jpg|right|thumb|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree Survey<ref>{{cite web |title=Dark matter may be smoother than expected – Careful study of large area of sky imaged by VST reveals intriguing result |url=https://www.eso.org/public/news/eso1642/ |access-date=8 December 2016 |website=www.eso.org}}</ref>]]Structure formation refers to the period after the [[Big Bang]] when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the [[FRW metric|Friedmann solutions]] to general relativity describe a homogeneous universe. Later, small [[Anisotropy|anisotropies]] gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.<ref name="Jaffe">{{cite web |author=Jaffe |first=A. H. |title=Cosmology 2012: Lecture Notes |url=http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-url=https://web.archive.org/web/20160717223916/http://astro.imperial.ac.uk/sites/default/files/cosmology.pdf |archive-date=July 17, 2016}}</ref> If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive [[potential well]] for ordinary matter collapsing later, speeding up the structure formation process.<ref name="Jaffe" /><ref>{{cite journal |author=Low |first=L. F. |date=12 October 2016 |title=Constraints on the composite photon theory |url=https://zenodo.org/record/896052 |journal=Modern Physics Letters A |volume=31 |issue=36 |page=1675002 |bibcode=2016MPLA...3175002L |doi=10.1142/S021773231675002X}}</ref>

=== Sky surveys and baryon acoustic oscillations ===
{{Main|Baryon acoustic oscillations}}
Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (~&nbsp;1%) preference for pairs of galaxies to be separated by 147&nbsp;Mpc, compared to those separated by 130–160&nbsp;Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the [[Sloan Digital Sky Survey]] and the [[2dF Galaxy Redshift Survey]].<ref>
{{cite journal |last1=Percival |first1=W. J. |display-authors=etal |year=2007 |title=Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=381 |issue=3 |pages=1053–1066 |arxiv=0705.3323 |bibcode=2007MNRAS.381.1053P |doi=10.1111/j.1365-2966.2007.12268.x |doi-access=free}}</ref> Combining the CMB observations with BAO measurements from galaxy [[redshift survey]]s provides a precise estimate of the [[Hubble's law|Hubble constant]] and the average matter density in the Universe.<ref name="Komatsu2009">{{cite journal |last1=Komatsu |first1=E. |display-authors=etal |year=2009 |title=Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation |journal=The Astrophysical Journal Supplement|volume=180 |issue=2 |pages=330–376 |arxiv=0803.0547 |bibcode=2009ApJS..180..330K |doi=10.1088/0067-0049/180/2/330|s2cid=119290314 }}</ref> The results support the Lambda-CDM model.

== Theoretical classifications ==
{{Main|Cold dark matter|Warm dark matter|Hot dark matter}}

Dark matter can be divided into ''cold'', ''warm'', and ''hot'' categories.<ref>{{cite book |first=Joseph |last=Silk |title=The Big Bang: Third Edition |chapter-url={{google books |plainurl=y |id=XLwe1lUmz5kC |page=82}} |date=2000 |publisher=Henry Holt and Company |isbn=978-0-8050-7256-3 |chapter=IX}}</ref> These categories refer to velocity rather than an actual temperature, and indicate how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion. This distance is called the ''[[free streaming]] length''. The categories of dark matter are set with respect to the size of the collection of mass prior to [[structure formation]] that later collapses to form a dwarf galaxy. This collection of mass is sometimes called a [[protogalaxy]]. Dark matter particles are classified as cold, warm, or hot if their free streaming length is much smaller (cold), similar to (warm), or much larger (hot) than the protogalaxy of a dwarf galaxy.<ref>{{cite book |last1=Bambi |first1=Cosimo |last2= D. Dolgov |first2=Alexandre |title=Introduction to Particle Cosmology |series=UNITEXT for Physics |year=2016 |language=English |publisher=Springer Berlin, Heidelberg | page=178 |doi=10.1007/978-3-662-48078-6 |isbn=978-3-662-48078-6}}</ref><ref>{{cite journal |first1=N. |last1=Vittorio |author2=J. Silk |date=1984 |journal=Astrophysical Journal Letters |volume=285 |pages=L39–L43 |title=Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter |doi=10.1086/184361 |bibcode=1984ApJ...285L..39V}}</ref><ref>{{cite journal |first1=Masayuki |last1=Umemura |author2=Satoru Ikeuchi |title=Formation of Subgalactic Objects within Two-Component Dark Matter |date=1985 |journal=Astrophysical Journal |volume=299 |pages=583–592 |doi=10.1086/163726 |bibcode=1985ApJ...299..583U}}</ref> Mixtures of the above are also possible: a theory of [[mixed dark matter]] was popular in the mid-1990s, but was rejected following the discovery of [[dark energy]].{{Citation needed|date=April 2016}}

The significance of the free streaming length is that the universe began with some primordial density fluctuations from the Big Bang (in turn arising from quantum fluctuations at the microscale). Particles from overdense regions will naturally spread to underdense regions, but because the universe is expanding quickly, there is a time limit for them to do so. Faster particles (hot dark matter) can beat the time limit while slower particles cannot. The particles travel a free streaming length's worth of distance within the time limit; therefore this length sets a minimum scale for later structure formation. Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies, while the reverse is true for cold dark matter.

[[List of deep fields|Deep-field observations]] show that galaxies formed first, followed by clusters and superclusters as galaxies clump together,<ref name="bertone hooper silk" /> and therefore that most dark matter is cold. This is also the reason why [[neutrinos]], which move at nearly the speed of light and therefore would fall under hot dark matter, cannot make up the bulk of dark matter.<ref name="Jaffe" />

{{multiple images |total_width=600 |direction=horizontal |align=center
 |image1=NASA-HubbleSpaceTelescope-DeepField-2017.jpg |image2=Webb's First Deep Field (adjusted).jpg
 |footer=[[Galaxy cluster]] [[SMACS J0723.3-7327]] observed with the [[Hubble Space Telescope]] (2017, left) and the [[James Webb Space Telescope]] (2022, right).<ref name="NASA-20220711">{{cite news |last=Garner |first=Rob |title=NASA's Webb Delivers Deepest Infrared Image of Universe Yet |url=https://www.nasa.gov/image-feature/goddard/2022/nasa-s-webb-delivers-deepest-infrared-image-of-universe-yet |date=11 July 2022 |work=[[NASA]] |access-date=12 July 2022 |archive-date=12 July 2022 |archive-url=https://web.archive.org/web/20220712000119/https://www.nasa.gov/image-feature/goddard/2022/nasa-s-webb-delivers-deepest-infrared-image-of-universe-yet/ |url-status=live }}</ref><ref name="NYT-20220711">{{cite news |last1=Overbye |first1=Dennis |last2=Chang |first2=Kenneth |last3=Tankersley |first3=Jim |title=Biden and NASA Share First Webb Space Telescope Image – From the White House on Monday, humanity got its first glimpse of what the observatory in space has been seeing: a cluster of early galaxies. |url=https://www.nytimes.com/2022/07/11/science/nasa-webb-telescope-images-livestream.html |date=11 July 2022 |work=[[The New York Times]] |access-date=12 July 2022 |archive-date=12 July 2022 |archive-url=https://web.archive.org/web/20220712005736/https://www.nytimes.com/2022/07/11/science/nasa-webb-telescope-images-livestream.html |url-status=live }}</ref><ref name="SA-202207">{{cite news |last=Pacucci |first=Fabio |title=How Taking Pictures of 'Nothing' Changed Astronomy - Deep-field images of "empty" regions of the sky from Webb and other space telescopes are revealing more of the universe than we ever thought possible |url=https://www.scientificamerican.com/article/how-taking-pictures-of-nothing-changed-astronomy1/ |date=15 July 2022 |work=[[Scientific American]] |access-date=16 July 2022 |archive-date=16 July 2022 |archive-url=https://web.archive.org/web/20220716023339/https://www.scientificamerican.com/article/how-taking-pictures-of-nothing-changed-astronomy1/ |url-status=live }}</ref><ref name="ABC-20220714">{{cite news |last1=Deliso |first1=Meredith |last2=Longo |first2=Meredith |last3=Rothenberg |first3=Nicolas |title=Hubble vs. James Webb telescope images: See the difference |url=https://abcnews.go.com/Technology/hubble-james-webb-telescope-images-difference/story?id=86763039 |date=14 July 2022 |work=[[ABC News (United States)|ABC News]] |access-date=15 July 2022 |archive-date=15 July 2022 |archive-url=https://web.archive.org/web/20220715003405/https://abcnews.go.com/Technology/hubble-james-webb-telescope-images-difference/story?id=86763039 |url-status=live }}</ref><ref name="CNET-20120713">{{cite news |last=Kooser |first=Amanda |title=Hubble and James Webb Space Telescope Images Compared: See the Difference - The James Webb Space Telescope builds on Hubble's legacy with stunning new views of the cosmos. |url=https://www.cnet.com/pictures/hubble-and-james-webb-space-telescope-images-compared-see-the-difference/ |date=13 July 2012 |work=[[CNET]] |access-date=16 July 2022 |archive-date=17 July 2022 |archive-url=https://web.archive.org/web/20220717015540/https://www.cnet.com/pictures/hubble-and-james-webb-space-telescope-images-compared-see-the-difference/ |url-status=live }}</ref><ref name="UT-20220502">{{cite news |last=Atkinson |first=Nancy |title=Now, We can Finally Compare Webb to Other Infrared Observatories |url=https://www.universetoday.com/155686/now-we-can-finally-compare-webb-to-other-infrared-observatories/ |date=2 May 2022 |work=[[Universe Today]] |access-date=12 May 2022 |archive-date=10 May 2022 |archive-url=https://web.archive.org/web/20220510035557/https://www.universetoday.com/155686/now-we-can-finally-compare-webb-to-other-infrared-observatories/ |url-status=live}}</ref> Both images show strong [[gravitational lensing]] features appearing as galaxies smeared into crescent shapes. JWST images show much higher sensitivity and resolution at infrared wavelengths, allowing it to see more distant, fainter objects in clearer detail.}}

== Composition ==
[[File:Dark matter candidates.pdf|thumb|upright=1.9|Different dark matter particle candidates by mass in electron volts (eV)]]

The identity of dark matter is unknown, but there are many [[hypothesis|hypotheses]] about what dark matter could consist of, as set out in the table below.
{| class = wikitable
|+ Major dark matter hypotheses
|-
|rowspan=3| Light bosons
| [[Axion]]s
|-
| [[WISP (particle physics)|Axion-like particles]]
|-
| [[Fuzzy cold dark matter]]
|-
|rowspan=2| [[Neutrino]]s
| [[neutrino|Standard Model]]{{efn|The three neutrino types already observed are indeed abundant, and dark, and matter, but their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from [[observable universe|large-scale structure]] and high-[[redshift]] galaxies.<ref name="bertone merritt" />}}
|-
| [[Sterile neutrinos]]
|-
|rowspan=5| Other particles
| [[Lightest supersymmetric particle]]
|-
| [[Weakly interacting massive particle]]s (WIMPs)
|-
| [[Self-interacting dark matter]]
|-
| [[Atomic dark matter]]<ref>{{cite journal |last1=Bansal |first1=Saurabh |last2=Barron |first2=Jared |last3=Curtin |first3=David |last4=Tsai |first4=Yuhsin |date=2023-10-16 |title=Precision cosmological constraints on atomic dark matter |journal=Journal of High Energy Physics |volume=2023 |issue=10 |page=95 |doi=10.1007/JHEP10(2023)095 |arxiv=2212.02487 |bibcode=2023JHEP...10..095B |issn=1029-8479}}</ref><ref>{{citation |last1=Bansal |first1=Saurabh |title=Precision Cosmological Constraints on Atomic Dark Matter |date=2023-07-27 |arxiv=2212.02487 |last2=Barron |first2=Jared |last3=Curtin |first3=David |last4=Tsai |first4=Yuhsin |journal=Journal of High Energy Physics |volume=2023 |issue=10 |page=95 |doi=10.1007/JHEP10(2023)095 |bibcode=2023JHEP...10..095B |quote=leading to a better fit than ΛCDM or ΛCDM + dark radiation}}</ref><ref>{{cite web |last=Sutter |first=Paul Sutter |date=2023-06-07 |title=Dark matter atoms may form shadowy galaxies with rapid star formation |url=https://www.space.com/dark-matter-atoms-form-stars-galaxies-simulations |access-date=2024-01-09 |website=Space.com |language=en}}</ref><ref name="MirrorStars">{{cite journal |last1=Armstrong |first1=Isabella |display-authors=etal |date=2024 |title=Electromagnetic Signatures of Mirror Stars |journal=The Astrophysical Journal |volume=965 |issue=1 |page=42 |arxiv=2311.18086 |bibcode=2024ApJ...965...42A |doi=10.3847/1538-4357/ad283c |doi-access=free}}</ref>
|-
| [[Strangelet]]<ref>{{cite journal |last1=VanDevender |first1=J. Pace |last2=VanDevender |first2=Aaron P. |last3=Sloan |first3=T. |last4=Swaim |first4=Criss |last5=Wilson |first5=Peter |last6=Schmitt |first6=Robert G. |last7=Zakirov |first7=Rinat |last8=Blum |first8=Josh |last9=Cross |first9=James L. |last10=McGinley |first10=Niall |date=2017-08-18 |title=Detection of magnetized quark-nuggets, a candidate for dark matter |journal=Scientific Reports |volume=7 |issue=1 |page=8758 |doi=10.1038/s41598-017-09087-3 |pmid=28821866 |pmc=5562705 |arxiv=1708.07490 |bibcode=2017NatSR...7.8758V |issn=2045-2322}}</ref>
|-
|rowspan=3| [[Macroscopic scale|Macroscopic]]
| [[Primordial black hole]]s (PBHs)<ref name="Carr24"/><ref name="Bird"/><ref name="jwst">{{cite journal |last1=Hütsi |first1=Gert |last2=Raidal |first2=Martti |last3=Urrutia |first3=Juan |last4=Vaskonen |first4=Ville |last5=Veermäe |first5=Hardi |date=2 February 2023 |title=Did JWST observe imprints of axion miniclusters or primordial black holes? |journal=Physical Review D |volume=107 |issue=4 |article-number=043502 |arxiv=2211.02651 |bibcode=2023PhRvD.107d3502H |doi=10.1103/PhysRevD.107.043502 |s2cid=253370365}}</ref><ref name="Carr">{{cite journal |last1=Carr |first1=Bernard |last2=Kühnel |first2=Florian |title=Primordial black holes as dark matter candidates |journal=SciPost Physics Lecture Notes |date=2 May 2022 |article-number=48 |doi=10.21468/SciPostPhysLectNotes.48 |s2cid=238407875 |url=https://scipost.org/SciPostPhysLectNotes.48/pdf |access-date=13 February 2023 |doi-access=free |arxiv=2110.02821 |archive-date=7 March 2023 |archive-url=https://web.archive.org/web/20230307155927/https://scipost.org/SciPostPhysLectNotes.48/pdf |url-status=live }} (See also the [https://indico.cern.ch/event/949654/contributions/4031007/attachments/2293539/3901659/Carr-Kuhnel.pdf accompanying slide presentation.] {{Webarchive|url=https://web.archive.org/web/20230213013128/https://indico.cern.ch/event/949654/contributions/4031007/attachments/2293539/3901659/Carr-Kuhnel.pdf |date=13 February 2023 }}</ref><ref name="Espinosa">{{cite journal |last1=Espinosa |first1=J. R. |last2=Racco |first2=D. |last3=Riotto |first3=A. |title=A Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matter |journal=Physical Review Letters |volume=120 |issue=12 |article-number=121301 |doi=10.1103/PhysRevLett.120.121301 |pmid=29694085 |date=23 March 2018|arxiv=1710.11196 |bibcode=2018PhRvL.120l1301E |s2cid=206309027 }}</ref><ref name="Clesse">{{cite journal |last1=Clesse |first1=Sebastien |last2=García-Bellido |first2=Juan |title=Seven Hints for Primordial Black Hole Dark Matter |journal=Physics of the Dark Universe |volume=22 |pages=137–146 |arxiv=1711.10458 |bibcode=2018PDU....22..137C |doi=10.1016/j.dark.2018.08.004 |year=2018 |s2cid=54594536 }}</ref><ref name="Lacki">{{cite journal |last1=Lacki |first1=Brian C. |last2=Beacom |first2=John F. |title=Primordial Black Holes as Dark Matter: Almost All or Almost Nothing |journal=The Astrophysical Journal |date=12 August 2010 |volume=720 |issue=1 |pages=L67–L71 |doi=10.1088/2041-8205/720/1/L67 |language=en |issn=2041-8205 |arxiv=1003.3466 |bibcode=2010ApJ...720L..67L |s2cid=118418220 }}</ref><ref name="Kashlinsky">{{cite journal |last1=Kashlinsky |first1=A. |title=LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies |journal=The Astrophysical Journal |date=23 May 2016 |volume=823 |issue=2 |pages=L25 |doi=10.3847/2041-8205/823/2/L25 |issn=2041-8213|arxiv=1605.04023 |bibcode=2016ApJ...823L..25K |s2cid=118491150 |doi-access=free }}</ref><ref name="Frampton">{{cite journal |last1=Frampton |first1=Paul H. |last2=Kawasaki |first2=Masahiro |last3=Takahashi |first3=Fuminobu |last4=Yanagida |first4=Tsutomu T. |title=Primordial Black Holes as All Dark Matter |journal=Journal of Cosmology and Astroparticle Physics |date=22 April 2010 |volume=2010 |issue=4 |page=023 |doi=10.1088/1475-7516/2010/04/023 |issn=1475-7516|arxiv=1001.2308 |bibcode=2010JCAP...04..023F |s2cid=119256778 }}</ref><ref name="Carneiro">{{cite journal |last1=Carneiro |first1=S. |last2=de Holanda |first2=P.C. |last3=Saa |first3=A. |title=Neutrino primordial Planckian black holes |journal=Physics Letters |date=2021 |volume=B822 |article-number=136670 |doi=10.1016/j.physletb.2021.136670 |issn=0370-2693|bibcode=2021PhLB..82236670C |s2cid=244196281 |doi-access=free |hdl=20.500.12733/1987 |hdl-access=free }}</ref>
|-
| Other [[massive compact halo objects]] (MACHOs)
|}

[[File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.ogv|thumb|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]] observations of dwarf galaxies provide new insights on dark matter.]]

=== Baryonic matter ===
{{Distinguish|Missing baryon problem}}

Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard [[baryon|baryonic matter]], such as protons or neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.<ref name=GianfracoHooperHistory>{{cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |title=History of dark matter |journal=Reviews of Modern Physics |date=15 October 2018 |volume=90 |issue=4 |article-number=045002 |doi=10.1103/RevModPhys.90.045002|arxiv=1605.04909 |bibcode=2018RvMP...90d5002B |s2cid=18596513 }}</ref><ref name=BaryonicSource01>{{cite web |url=http://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter |title=Baryonic Matter |website=COSMOS – The SAO Encyclopedia of Astronomy |publisher=[[Swinburne University of Technology]]|access-date=16 November 2022}}</ref> A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost.<ref>{{cite web |title=Baryonic Matter |url=https://astronomy.swin.edu.au/cosmos/b/Baryonic+Matter |access-date=2023-10-03 |website=astronomy.swin.edu.au |publisher=Cosmos: The Swinburne Astronomy Online Encyclopedia |publication-place=Melbourne, Victoria, Australia: Swinburne University of Technology}}</ref> 

These massive objects that are hard to detect are collectively known as [[Massive compact halo object|MACHO]]s. Some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.<ref name=Randall_2015/>{{rp|286}}<ref>{{cite news |title=MACHOs may be out of the running as a dark matter candidate |url=https://astronomy.com/news/2016/08/machos-may-be-out-of-the-running-as-a-dark-matter-candidate |access-date=16 November 2022 |work=Astronomy.com |date=2016 |language=en}}</ref>

However, multiple lines of evidence suggest the majority of dark matter is not baryonic:
* Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
* The theory of [[Big Bang nucleosynthesis]] predicts the observed [[abundance of the chemical elements]]. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.<ref>{{cite book |author=Weiss, Achim |url=http://www.einstein-online.info/spotlights/BBN |title=Big bang nucleosynthesis: Cooking up the first light elements |archive-url=https://web.archive.org/web/20130206021217/http://www.einstein-online.info/spotlights/BBN |archive-date=6 February 2013 |publisher=Einstein Online |volume=2 |year=2006 |page=1017 |access-date=1 June 2013 }}</ref><ref>{{cite book |last1=Raine |first1=D. |last2=Thomas |first2=T. |date=2001 |title=An Introduction to the Science of Cosmology |page=30 |publisher=[[IOP Publishing]] |isbn=978-0-7503-0405-4 |oclc=864166846 }}</ref> Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's [[Friedmann equations#Density parameter|critical density]]. In contrast, [[large-scale structure of the universe|large-scale structure]] and other observations indicate that the total matter density is about 30% of the critical density.<ref name="planckesa2015" />
* Astronomical searches for [[gravitational microlensing]] in the [[Milky Way]] found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30&nbsp;solar masses, which covers nearly all the plausible candidates.<ref>{{cite journal |last1=Tisserand |first1=P. |last2=Le&nbsp;Guillou |first2=L. |last3=Afonso |first3=C. |last4=Albert |first4=J.N. |last5=Andersen |first5=J. |last6=Ansari |first6=R. |last7=Aubourg |first7=É. |last8=Bareyre |first8=P. |last9=Beaulieu |first9=J.P. |last10=Charlot |first10=X. |last11=Coutures |first11=C. |last12=Ferlet |first12=R. |last13=Fouqué |first13=P. |last14=Glicenstein |first14=J.F. |last15=Goldman |first15=B. |last16=Gould |first16=A. |last17=Graff |first17=D. |last18=Gros |first18=M. |last19=Haissinski |first19=J. |last20=Hamadache |first20=C. |last21=De Kat |first21=J. |last22=Lasserre |first22=T. |last23=Lesquoy |first23=É. |last24=Loup |first24=C. |last25=Magneville |first25=C. |last26=Marquette |first26=J.B. |last27=Maurice |first27=É. |last28=Maury |first28=A. |last29=Milsztajn |first29=A. |last30=Moniez |first30=M. |display-authors=6 |title=Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=469 |issue=2 |pages=387–404 |year=2007 |doi=10.1051/0004-6361:20066017 |url=https://www.researchgate.net/publication/41714676 |arxiv=astro-ph/0607207 |bibcode=2007A&A...469..387T|s2cid=15389106 }}</ref><ref>{{cite journal |last1=Graff |first1=D. S. |last2=Freese |first2=K. |year=1996 |title=Analysis of a ''Hubble Space Telescope'' Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo |journal=The Astrophysical Journal |volume=456 |issue=1996 |page=L49 |arxiv=astro-ph/9507097 |bibcode=1996ApJ...456L..49G |doi=10.1086/309850 |s2cid=119417172}}</ref><ref>{{cite journal |last1=Najita |first1=J. R. |last2=Tiede |first2=G. P. |last3=Carr |first3=J. S. |year=2000 |title=From Stars to Superplanets: The Low-Mass Initial Mass Function in the Young Cluster IC 348 |journal=The Astrophysical Journal |volume=541 |issue=2 |pages=977–1003 |arxiv=astro-ph/0005290 |bibcode=2000ApJ...541..977N |doi=10.1086/309477 |s2cid=55757804}}</ref><ref>{{cite journal |arxiv=1106.2925 |title=The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=4 |pages=2949–2961 |last1=Wyrzykowski |first1=L. |last2=Skowron |first2=J. |last3=Kozlowski |first3=S. |last4=Udalski |first4=A. |last5=Szymanski |first5=M.K. |last6=Kubiak |first6=M. |last7=Pietrzynski |first7=G. |last8=Soszynski |first8=I. |last9=Szewczyk |first9=O. |last10=Ulaczyk |first10=K. |last11=Poleski |first11=R. |last12=Tisserand |first12=P. |display-authors=6 |doi=10.1111/j.1365-2966.2011.19243.x |year=2011 |doi-access=free |bibcode=2011MNRAS.416.2949W|s2cid=118660865 }}</ref><ref>{{cite arXiv |title=Death of stellar baryonic dark matter candidates |first1=Katherine |last1=Freese |eprint=astro-ph/0007444 |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2000 }}</ref><ref>{{cite book |pages=4–6 |first1=Katherine |last1=Freese |arxiv=astro-ph/0002058 |bibcode=2000fist.conf...18F |doi=10.1007/10719504_3 |chapter=Death of Stellar Baryonic Dark Matter |series=ESO Astrophysics Symposia |last2=Fields |first2=Brian |last3=Graff |first3=David |year=2003 |title=The First Stars |isbn=978-3-540-67222-7 |citeseerx=10.1.1.256.6883|s2cid=119326375 }}</ref>
* Detailed analysis of the small irregularities (anisotropies) in the [[cosmic microwave background]] by [[WMAP]] and [[Planck (spacecraft)|Planck]] indicate that around five-sixths of the total matter is in a form that only interacts significantly with ordinary matter or [[photon]]s through gravitational effects.<ref>{{cite journal |first1=L. |last1=Canetti |first2=M. |last2=Drewes |first3=M. |last3=Shaposhnikov |title=Matter and Antimatter in the Universe |journal=New J. Phys. |year=2012 |volume=14 |issue=9 |article-number=095012 |doi=10.1088/1367-2630/14/9/095012 |arxiv=1204.4186 |bibcode=2012NJPh...14i5012C|s2cid=119233888 }}</ref>

=== Non-baryonic matter ===
There are two main candidates for non-baryonic dark matter: new particles and [[primordial black hole]]s. Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the [[Chemical element|elements]] in the early universe ([[Big Bang nucleosynthesis]])<ref>{{Cite journal |last1=Kernan |first1=Peter J. |last2=Krauss |first2=Lawrence M. |date=1994-05-23 |title=Refined big bang nucleosynthesis constraints on Ω B and N ν |url=https://link.aps.org/doi/10.1103/PhysRevLett.72.3309 |journal=Physical Review Letters |volume=72 |issue=21 |pages=3309–3312 |doi=10.1103/PhysRevLett.72.3309 |pmid=10056165 |issn=0031-9007|arxiv=astro-ph/9402010 }}</ref><ref>{{Cite journal |last1=Smith |first1=Michael S. |last2=Kawano |first2=Lawrence H. |last3=Malaney |first3=Robert A. |date=1993 |title=Experimental, computational, and observational analysis of primordial nucleosynthesis |url=http://adsabs.harvard.edu/doi/10.1086/191763 |journal=The Astrophysical Journal Supplement Series |volume=85 |page=219 |doi=10.1086/191763 |bibcode=1993ApJS...85..219S |issn=0067-0049}}</ref><ref name="Copi 1995" /> and so its presence is felt only via its gravitational effects (such as [[weak lensing]]). In addition, some dark matter candidates can interact with themselves ([[self-interacting dark matter]]) or with ordinary particles (e.g. [[Weakly Interacting Massive Particle|WIMP]]s), possibly resulting in observable by-products such as [[gamma rays]] and neutrinos (indirect detection).<ref name="bertone merritt">{{cite journal |last1=Bertone |first1=G. |last2=Merritt |first2=D. |title=Dark Matter Dynamics and Indirect Detection |year=2005 |journal=[[Modern Physics Letters A]] |volume=20 |issue=14 |pages=1021–1036 |arxiv=astro-ph/0504422 |bibcode=2005MPLA...20.1021B |doi=10.1142/S0217732305017391|s2cid=119405319 }}</ref> Candidates abound (see the table above), each with their own strengths and weaknesses.

==== Particle candidates ====
===== Weakly Interacting Massive Particles =====
[[File:WIMPsLZexperiment2023.png|Upper limits for WIMP-nucleon elastic cross sections from selected experiments as reported by the LZ experiment in July 2023.|thumb]]
{{main|Weakly Interacting Massive Particle}}
There exists no formal definition of a Weakly Interacting Massive Particle (WIMP), but broadly, it is an [[elementary particle]] which interacts via [[gravity]] and any other force (or forces) which is as weak as or weaker than the [[weak nuclear force]], but also non-vanishing in strength. Many WIMP candidates are expected to have been produced thermally in the early Universe, similarly to the particles of the Standard Model<ref>{{cite journal | last = Garrett | first = Katherine | title = Dark matter: A primer | year = 2010 | journal = Advances in Astronomy | volume = 2011 | issue = 968283 | pages = 1–22 | doi = 10.1155/2011/968283| arxiv = 1006.2483 | bibcode = 2011AdAst2011E...8G | doi-access = free }}</ref> according to [[Big Bang]] cosmology, and usually will constitute [[cold dark matter]]. Obtaining the correct abundance of dark matter today via [[thermal production]] requires a self-[[annihilation]] [[Cross section (physics)|cross section]] of <math>\langle \sigma v \rangle</math> ≃ {{val|3|e=-26|u=cm<sup>3</sup>⋅s<sup>−1</sup>}}, which is roughly what is expected for a new particle in the 100&nbsp;[[GeV]]/''c''<sup>2</sup> mass range that interacts via the [[electroweak force]].

Because [[supersymmetry|supersymmetric]] extensions of the Standard Model of particle physics readily predict a new particle with these properties, this apparent coincidence has been called the "WIMP miracle", and a stable supersymmetric partner has long been a prime explanation for dark matter.<ref>{{cite journal |last1=Jungman |first1=Gerard |last2=Kamionkowski |first2=Marc |last3=Griest |first3=Kim |year=1996 |title=Supersymmetric dark matter |journal=Physics Reports |volume=267 |issue=5–6 |pages=195–373 |s2cid=119067698 |arxiv=hep-ph/9506380 |bibcode=1996PhR...267..195J |doi=10.1016/0370-1573(95)00058-5}}</ref> Experimental efforts to detect WIMPs include the search for products of WIMP annihilation, including [[gamma ray]]s, [[neutrino]]s and [[cosmic ray]]s in nearby galaxies and galaxy clusters; direct detection experiments designed to measure the collision of WIMPs with [[Atomic nucleus|nuclei]] in the laboratory, as well as attempts to directly produce WIMPs in colliders, such as the [[Large Hadron Collider]] at [[CERN]]. In the early 2010s, results from [[#Direct detection|direct-detection]] experiments along with the lack of evidence for supersymmetry at the [[Large Hadron Collider]] (LHC) experiment<ref>{{cite news |url=http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm |title=LHC discovery maims supersymmetry again |website=Discovery News |archive-date=13 March 2016 |access-date=16 January 2025 |archive-url=https://web.archive.org/web/20160313000505/http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm }}</ref><ref>{{cite arXiv |last=Craig |first=Nathaniel |year=2013 |title=The State of Supersymmetry after Run I of the LHC |class=hep-ph |eprint=1309.0528}}</ref> have cast doubt on the simplest WIMP hypothesis.<ref>{{cite journal |last1=Fox |first1=Patrick J. |last2=Jung |first2=Gabriel |last3=Sorensen |first3=Peter |last4=Weiner |first4=Neal |year=2014 |title=Dark matter in light of LUX |journal=Physical Review D |volume=89 |issue=10 |article-number=103526 |arxiv=1401.0216 |bibcode=2014PhRvD..89j3526F |doi=10.1103/PhysRevD.89.103526}}</ref>

===== Axions =====
{{main|Axion}}

Axions are hypothetical elementary particles originally theorized in 1978 independently by [[Frank Wilczek]] and [[Steven Weinberg]] as the [[Goldstone boson]] of [[Peccei–Quinn theory]], which had been proposed in 1977 to solve the [[strong CP problem]] in [[quantum chromodynamics]] (QCD). QCD effects produce an effective periodic potential in which the axion field moves.<ref name="peccei2008">{{cite book|last=Peccei | first=R. D. | title=Axions: Theory, Cosmology, and Experimental Searches |year=2008 |chapter=The Strong CP Problem and Axions |editor1-last=Kuster |editor1-first=Markus |editor2-last=Raffelt |editor2-first=Georg |editor3-last=Beltrán |editor3-first=Berta |series=Lecture Notes in Physics |volume=741 |pages=3–17 |arxiv=hep-ph/0607268 |doi=10.1007/978-3-540-73518-2_1 |isbn=978-3-540-73517-5|s2cid=119482294 }}</ref> Expanding the potential about one of its minima, one finds that the product of the axion mass with the axion decay constant is determined by the topological susceptibility of the QCD vacuum. An axion with mass that is much less than 60&nbsp;keV/''c''<sup>2</sup> is long-lived and weakly interacting: a perfect dark matter candidate.

The oscillations of the axion field about the minimum of the effective potential, the so-called misalignment mechanism, generate a cosmological population of cold axions with an abundance depending on the mass of the axion.<ref name="auto">{{cite journal |last1=Preskill |first1=J. |author1-link=John Preskill |last2=Wise |first2=M. |author2-link=Mark B. Wise |last3=Wilczek |first3=F. |author3-link=Frank Wilczek |date=6 January 1983 |journal=Physics Letters&nbsp;B |volume=120 |issue=1–3 |pages=127–132 |title=Cosmology of the invisible axion |doi=10.1016/0370-2693(83)90637-8 |bibcode=1983PhLB..120..127P |url=http://www.theory.caltech.edu/~preskill/pubs/preskill-1983-axion.pdf |citeseerx=10.1.1.147.8685 |archive-date=28 May 2016 |access-date=16 January 2025 |archive-url=https://web.archive.org/web/20160528001653/http://www.theory.caltech.edu/~preskill/pubs/preskill-1983-axion.pdf |url-status=live }}</ref><ref name="A cosmological bound on the invisib">{{cite journal |last1=Abbott |first1=L. |last2=Sikivie |first2=P. |year=1983 |journal=Physics Letters&nbsp;B |volume=120 |issue=1–3 |pages=133–136 |title=A cosmological bound on the invisible axion |bibcode=1983PhLB..120..133A |doi=10.1016/0370-2693(83)90638-X |citeseerx=10.1.1.362.5088}}</ref><ref name="The not-so-harmless axion">{{cite journal |last1=Dine |first1=M. |last2=Fischler |first2=W. |year=1983 |journal=Physics Letters B |volume=120 |issue=1–3 |pages=137–141 |title=The not-so-harmless axion |doi=10.1016/0370-2693(83)90639-1 |bibcode=1983PhLB..120..137D}}</ref> With a mass above 5&nbsp;[[electron-volt|μeV/{{mvar|c}}<sup>2</sup>]] ({{10^|−11}} times the [[electron mass]]) axions could account for dark matter, and thus be both a dark-matter candidate and a solution to the strong CP problem. If inflation occurs at a low scale and lasts sufficiently long, the axion mass can be as low as 1&nbsp;peV/''c''<sup>2</sup>.<ref>{{cite journal |last1=di Luzio |first1=L. |last2=Nardi |first2=E. |last3=Giannotti |first3=M. |last4=Visinelli |first4=L. |date=25 July 2020 |journal=Physics Reports |volume=870 |pages=1–117 |title=The landscape of QCD axion models |bibcode=2020PhR...870....1D |doi=10.1016/j.physrep.2020.06.002 |arxiv=2003.01100 |s2cid=211678181 }}</ref><ref>{{cite journal |last1=Graham |first1=Peter W. |last2=Scherlis |first2=Adam |title=Stochastic axion scenario |journal=Physical Review D |date=9 August 2018 |volume=98 |issue=3 |article-number=035017 |doi=10.1103/PhysRevD.98.035017 |arxiv=1805.07362 |bibcode=2018PhRvD..98c5017G |s2cid=119432896 }}</ref><ref>{{cite journal |last1=Takahashi |first1=Fuminobu |last2=Yin |first2=Wen |last3=Guth |first3=Alan H. |title=The QCD Axion Window and Low Scale Inflation |journal=Physical Review D |date=31 July 2018 |volume=98 |issue=1 |article-number=015042 |doi=10.1103/PhysRevD.98.015042 |arxiv=1805.08763 |bibcode=2018PhRvD..98a5042T |s2cid=54584447 }}</ref>

[[File:Conceptual-arrangement-of-an-enhanced-axion-helioscope-with-x-ray-focalization-Solar.png|thumb|Principle of operation of the IAXO/BabyIAXO helioscope experiment for detecting axions]]

Because axions have extremely low mass, their [[de Broglie wavelength]] is very large, in turn meaning that quantum effects could help resolve the small-scale problems of the [[Lambda-CDM]] model. A single ultralight axion with a decay constant at the [[grand unified theory]] scale provides the correct relic density without fine-tuning.<ref>{{cite journal |last=Marsh |first=David J.E. |date=2016 |title=Axion cosmology |journal=Physics Reports |volume=643 |pages=1–79 |doi=10.1016/j.physrep.2016.06.005 |arxiv=1510.07633|bibcode=2016PhR...643....1M |s2cid=119264863 }}</ref> Axions as a dark matter candidate have gained in popularity in recent years, because of the non-detection of WIMPs.<ref>{{cite web|url=https://physicsworld.com/a/dark-matters-secret-identity-wimps-or-axions/?form=MG0AV3|title=Dark matter's secret identity: WIMPs or axions?|publisher=Physics World|date = 25 June 2024}}</ref>

===== Particle aggregation and dense dark matter objects =====
If dark matter is composed of weakly interacting particles, then an obvious question is whether it can form objects equivalent to [[planet]]s, [[star]]s, or [[black hole]]s. Historically, the answer has been it cannot,{{efn
|"One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) {{nobr|dark matter." — Buckley & Difranzo (2018)<ref name=curio/>}}
}}<ref name=curio>
{{cite journal
 |last1=Buckley |first1=Matthew R.
 |last2=Difranzo |first2=Anthony
 |date=1 February 2018
 |title=Synopsis: A way to cool dark matter
 |journal=[[Physical Review Letters]]
 |volume=120  |issue=5  |article-number=051102
 |doi=10.1103/PhysRevLett.120.051102
 |bibcode=2018PhRvL.120e1102B  |pmid=29481169
 |arxiv=1707.03829  |s2cid=3757868
 |url=https://physics.aps.org/articles/v11/s15
 |archive-url=https://archive.today/20201026224145/https://physics.aps.org/articles/v11/s15
 |archive-date=26 October 2020
}}
</ref><ref name=cornell_ask>{{cite web
 |title=Are there any dark stars or dark galaxies made of dark matter?

 |department=Ask an Astronomer

 |website=curious.astro.cornell.edu

 |publisher=[[Cornell University]]

 |url=https://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced%0A%C2%A0
 |archive-url=https://web.archive.org/web/20150302105015/http://curious.astro.cornell.edu/about-us/95-the-universe/galaxies/general-questions/508-are-there-any-dark-stars-or-dark-galaxies-made-of-dark-matter-advanced
 |archive-date=2 March 2015
 |access-date=23 January 2026
 |url-status=live
 }}</ref><ref name=siegel>
{{cite magazine
 |author-link=Ethan Siegel |author=Siegel, Ethan
 |date=28 October 2016
 |title=Why doesn't dark matter form black holes?
 |magazine=[[Forbes (magazine)|Forbes]]
 |url=https://www.forbes.com/sites/startswithabang/2016/10/28/why-doesnt-dark-matter-form-black-holes/#4e5014943de1
}}
</ref> because of two factors:
* It lacks an efficient means to lose energy<ref name=curio/> 
: Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase [[velocity]] and [[momentum]]. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The [[virial theorem]] suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.
* It lacks a diversity of interactions needed to form structures<ref name=siegel/>
: Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of [[neutrino]]s and [[electromagnetic radiation]] through [[nuclear fusion|fusion]] when they become energetic enough. [[Proton]]s and [[neutron]]s can bind via the [[strong interaction]] and then form [[atom]]s with [[electron]]s largely through [[electromagnetic interaction]]. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the [[weak interaction]], although until dark matter is better understood, this is only speculation).

==== Primordial black holes ====
[[File:PBHs-formation.png|upright=2|thumb|Formation of the universe without (above) and with (below) primordial black holes]]
{{main|Primordial black hole}}
{{See also|Massive compact halo object}}

Primordial black holes (PBHs) are hypothetical [[black hole]]s that formed soon after the [[Big Bang]]. In the [[inflationary era]] and early [[Scale factor (cosmology)#Radiation-dominated era|radiation-dominated]] universe, extremely dense pockets of [[Subatomic particle|subatomic matter]] may have been tightly packed to the point of [[gravitational collapse]], creating black holes without the [[supernova]] compression typically needed to create [[stellar black hole]]s.<ref name="Bird" /> The idea was first suggested by [[Yakov Zeldovich]] and [[Igor Dmitriyevich Novikov]] in 1966,<ref>{{cite journal|last1=Zel'dovitch & Novikov|title=The Hypothesis of Cores Retarded During Expansion and the Hot Cosmological Model|journal=Soviet Astronomy|date=14 March 1966|volume=10|issue=4|pages=602–603|bibcode=1966AZh....43..758Z}}</ref> and independently by [[Stephen Hawking]] in 1971.<ref>{{cite journal |last1=Hawking |first1=Stephen W. |year=1971 |title=Gravitationally collapsed objects of very low mass |journal=Mon. Not. R. Astron. Soc. |volume=152 |page=75 |bibcode=1971MNRAS.152...75H |doi=10.1093/mnras/152.1.75 |doi-access=free}}</ref> Because PBHs would form prior to [[stellar evolution]], they are non-[[baryonic dark matter]] candidates and are not limited to the narrow mass range of stellar black holes; they could range from Planck-mass relics to supermassive scales.<ref name="Carr24" />

Interest in PBHs as a primary component of dark matter was revived following the 2015 discovery of [[gravitational wave]]s by [[LIGO]]. Their first detected merger involved black holes of approximately 30 [[solar mass]]es; such objects are difficult to explain via standard stellar collapse but fit the predicted mass range for PBHs formed during the [[QCD]] transition in the early universe.<ref name="Bird" /> This interest was bolstered in November 2025, when the LIGO/Virgo/KAGRA collaboration reported a candidate gravitational wave signal from a sub-solar mass merger. As no astrophysical process is known to produce black holes below the [[Chandrasekhar limit]] (~1.4 solar masses), confirmed sub-solar mass objects would be strong evidence for a primordial origin.<ref>{{cite web |last=Cho |first=Adrian |date=2025-11-18 |title=Curious gravitational wave may be hint at primordial black holes—or just be noise |url=https://www.science.org/content/article/curious-gravitational-wave-may-be-hint-primordial-black-holes-or-just-noise |website=Science |publisher=American Association for the Advancement of Science |access-date=14 December 2025}}</ref><ref>{{cite web |last=Carpineti |first=Alfredo |date=2025-11-17 |title=Candidate gravitational wave detection hints at first-of-its-kind incredibly small object |url=https://www.iflscience.com/candidate-gravitational-wave-detection-hints-at-first-of-its-kind-incredibly-small-object-81582 |website=IFLScience |access-date=December 14, 2025}}</ref><ref>{{cite web |title=GCN Circular 42650 |url=https://gcn.nasa.gov/circulars/42650 |author=LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration |date=November 2025 |work=[[General Coordinates Network]] |publisher=[[NASA]] |quote=The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses....}}</ref> As [[List of gravitational wave observations|there have been no gravitational waves detected at z>1 (>6 Gya)]], and the sensitivity to lower-mass collisions falls off with distance, we are not currently able to detect collisions in the earliest half of the age of the universe.<ref>{{cite journal |last1=Abbott |first1=R. |display-authors=et al. |collaboration=LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration |title=The population of merging compact binaries inferred using gravitational waves through GWTC-3 |journal=[[Physical Review X]] |volume=13 |issue=1 |article-number=011048 |year=2023 |doi=10.1103/PhysRevX.13.011048 |arxiv=2111.03634 |bibcode=2023PhRvX..13a1048A |url=https://journals.aps.org/prx/abstract/10.1103/PhysRevX.13.011048 |access-date=January 13, 2026}}</ref>

[[File:Webb witnesses a feasting supermassive black hole in the early Universe (54933056627).jpg|thumb|left|November 2025 JWST observations confirmed an actively growing supermassive black hole within a "[[Little red dot (astronomical object)|little red dot]]" galaxy named CANUCS-LRD-z8.6.<ref>{{cite journal |last=Tripodi |first=Roberta |display-authors=et al. |date=November 19, 2025 |title=Extreme properties of a compact and massive accreting black hole host in the first 500 Myr |journal=[[Nature Communications]] |volume=16 |issue=1 |article-number=9830 |doi=10.1038/s41467-025-65070-x |doi-access=free |pmid=41257816 |pmc=12630725 |arxiv=2412.04983 |bibcode=2025NatCo..16.9830T }}</ref>]]

Further support for the PBH hypothesis has emerged from [[James Webb Space Telescope]] (JWST) observations of the high-[[redshift]] universe (z > 7). JWST discovered unexpected populations of "[[Little red dot (astronomical object)|Little Red Dots]]" (LRDs, compact very high redshift objects) and "overmassive black hole galaxies" such as [[UHZ1]] and [[GHZ2]], which contain supermassive black holes appearing less than 500 million years after the Big Bang and outweighing their galaxy's stars.<ref name=early1>{{cite arXiv |last=Juodžbalis |first=Ignas |display-authors=et al. |date=August 29, 2025 |title=A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization |eprint=2508.21748 |class=astro-ph.GA}}</ref><ref name=earliest>{{cite arXiv |last=Chavez Ortiz |first=Oscar A. |display-authors=et al. |date=November 4, 2025 |title=Significant Evidence of an AGN Contribution in GHZ2 at z = 12.34 |eprint=2511.03035 |class=astro-ph.GA}}</ref> These [[active galactic nuclei]] challenge standard models of accretion from "light" stellar black hole seeds, and suggest "heavy seeds" formed via [[Direct collapse black hole|direct collapse]] or PBHs, which could account for a significant fraction of dark matter halos.<ref name="jwst1">{{Cite journal |last1=Liu |first1=Boyuan |last2=Bromm |first2=Volker |date=September 27, 2022 |title=Accelerating Early Massive Galaxy Formation with Primordial Black Holes |journal=The Astrophysical Journal Letters |volume=937 |issue=2 |pages=L30 |doi=10.3847/2041-8213/ac927f |arxiv=2208.13178 |bibcode=2022ApJ...937L..30L |s2cid=252355487 |issn=2041-8205 |doi-access=free }}</ref>

Various observational constraints, such as [[gravitational microlensing]] data from the [[Subaru Telescope]] (HSC) <ref>{{cite journal |last=Niikura |first=Hiroko |date=1 April 2019 |title=Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations |journal=[[Nature Astronomy]] |volume=3 |issue=6 |pages=524–534 |doi=10.1038/s41550-019-0723-1 |s2cid=118986293 |bibcode=2019NatAs...3..524N |arxiv=1701.02151 }}</ref> and from ''[[Voyager 1]]'' electron-positron data <ref>{{cite journal |last1=Boudaud |first1=Mathieu |last2=Cirelli |first2=Marco |date=14 January 2019 |title=Voyager 1 [electron and position data] Further Constrain Primordial Black Holes as Dark Matter |journal=[[Physical Review Letters]] |volume=122 |issue=4 |pages=041104–041109 |doi=10.1103/PhysRevLett.122.041104 |pmid=30768336 |arxiv=1807.03075 }}</ref> have ruled out PBHs constituting 100% of dark matter in specific mass windows (e.g., evaporating tiny black holes or monochromatic intermediate-mass populations).
However, those constraints assume all PBHs have the same mass, a monochromatic mass distribution. More recent analyses utilizing extended mass distributions, predicted by inflation models and evident in gravitational wave and JWST observations, remove such constraints. A 2024 review indicates that PBHs with a broad, [[platykurtic]] mass distribution peaking around one solar mass could explain the entirety of dark matter, or coexist with other candidates in a mixed dark matter scenario.<ref name="Carr24" /><ref name="Villanueva2021">{{cite journal |last1=Villanueva-Domingo|first1=Pablo|last2=Mena|first2=Olga|last3=Palomares-Ruiz|first3=Sergio|title=A Brief Review on Primordial Black Holes as Dark Matter|journal=Frontiers in Astronomy and Space Sciences|date=2021 |volume=8 |page=87 |article-number=681084 |doi=10.3389/fspas.2021.681084|doi-access=free |arxiv=2103.12087 |bibcode=2021FrASS...8...87V }}</ref>

===== Fine tuning issues =====
[[File:PBH formation.png|upright=1.45|thumb|Primordial black holes were possibly formed by the collapse of overdense regions in the inflationary or early radiation-dominated universe.<ref>{{Cite journal |last1=Kawasaki |first1=Masahiro |last2=Kitajima |first2=Naoya |last3=Yanagida |first3=Tsutomu T. |date=March 18, 2013 |title=Primordial black hole formation from an axionlike curvaton model |journal=Physical Review D |volume=87 |issue=6 |article-number=063519 |doi=10.1103/PhysRevD.87.063519|arxiv=1207.2550 |bibcode=2013PhRvD..87f3519K |s2cid=119230374 }}</ref>]]

The primary theoretical challenge to the PBH hypothesis is the physical mechanism of their formation. Standard models of [[cosmic inflation]], known as "slow-roll inflation", generate density fluctuations that are far too small to trigger primordial collapse. Consequently, producing the required abundance of PBHs typically necessitates "exotic" inflation models, often featuring [[inflection point]]s, bumps, or plateaus in the [[Inflaton|inflaton potential]], which can amplify fluctuations by orders of magnitude.<ref>{{cite journal |last1=Green |first1=Anne M. |last2=Kavanagh |first2=Bradley J. |title=Primordial Black Holes as a Dark Matter Candidate |journal=[[Journal of Physics G: Nuclear and Particle Physics]] |volume=48 |issue=4 |page=043001 |date=2021 |doi=10.1088/1361-6471/abc534 |arxiv=2007.10722 |bibcode=2021JPhG...48d3001G }}</ref> Critics argue that these models require significant [[Fine-tuning (physics)|fine-tuning]], as the resulting PBH abundance is exponentially sensitive to the amplitude of these fluctuations; meaning that a slight deviation in parameters results in either a negligible amount of dark matter or a universe dominated entirely by black holes.<ref name="Villanueva2021" /><ref name="Lacki" /> However, proponents contend that as the natural parameter space for WIMPs is increasingly excluded by null results from all detection experiments, particle dark matter theories now require comparable levels of fine-tuning. Furthermore, proponents argue that the specific mass structures predicted by these exotic inflation models provide a unified explanation for observational anomalies seen by LIGO and JWST that particle models do not address.<ref name="Carr24" />

To address the fine-tuning problem, recent research has focused on mechanisms that generate the required fluctuations through natural physical processes rather than manual adjustments to the inflaton potential. One such mechanism is the [[QCD phase transition]]; as the universe cooled through this epoch, the reduction in the [[Equation of state (cosmology)|equation of state]] (pressure) naturally lowered the threshold for gravitational collapse. This effect automatically enhances the formation of black holes at the solar mass scale, comparable to those detected by gravitational wave observatories, without requiring a precisely tuned peak in the inflation power spectrum.<ref>{{cite journal |last1=Byrnes |first1=Christian T. |last2=Hindmarsh |first2=Mark |last3=Young |first3=Sam |last4=Hawkins |first4=Michael R. S. |title=Primordial Black Holes with an Accurate QCD Equation of State |journal=[[Journal of Cosmology and Astroparticle Physics]] |volume=2018 |issue=8 |page=041 |date=2018 |doi=10.1088/1475-7516/2018/08/041 |arxiv=1801.06138 |bibcode=2018JCAP...08..041B }}</ref> Additionally, models involving multiple [[scalar fields]] can produce sharp spikes in density fluctuations through dynamic interactions, such as rapid turns in the field trajectory, which derive the necessary conditions from the model's geometric structure rather than from fine-tuned parameters.<ref>{{cite journal |last1=Palma |first1=Gonzalo A. |last2=Sypsas |first2=Spyros |last3=Zenteno |first3=Cristóbal |title=Seeding Primordial Black Holes in Multi-Field Inflation |journal=[[Physical Review Letters]] |volume=125 |issue=12 |article-number=121301 |date=2020 |doi=10.1103/PhysRevLett.125.121301 |pmid=33016764 |arxiv=2004.06106 |bibcode=2020PhRvL.125l1301P }}</ref>

== Particle searches ==
If dark matter is made up of subatomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.<ref name="Gaitskell 2004">{{cite journal |last=Gaitskell |first=Richard J. |s2cid=11316578 |title=Direct Detection of Dark Matter |journal=[[Annual Review of Nuclear and Particle Science]] |volume=54 |pages=315–359 |bibcode=2004ARNPS..54..315G |date=2004 |doi=10.1146/annurev.nucl.54.070103.181244|doi-access=free}}</ref><ref name="Number per second">{{cite web |title=Neutralino Dark Matter |url=http://www.picassoexperiment.ca/dm_neutralino.php |access-date=26 December 2011}}
{{cite web |title=WIMPs and MACHOs |url=http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-url=https://web.archive.org/web/20060923123531/http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf |archive-date=2006-09-23 |url-status=live |last=Griest |first=Kim |access-date=26 December 2011}}</ref> Many experiments aim to test this hypothesis. Although WIMPs have been the main search candidates,<ref name="bertone hooper silk" /> [[axion]]s have drawn renewed attention, with the [[Axion Dark Matter Experiment]] (ADMX) searches for axions and many more planned in the future.<ref name="Chadha-Day et al">{{cite journal |last1=Chadha-Day |first1=Francesca |last2=Ellis |first2=John |last3=Marsh |first3=David J. E. |date=23 February 2022 |title=Axion dark matter: What is it and why now? |journal=Science Advances |volume=8 |issue=8 |article-number=eabj3618 |arxiv=2105.01406 |bibcode=2022SciA....8J3618C |doi=10.1126/sciadv.abj3618 |pmc=8865781 |pmid=35196098}}</ref> Another candidate is heavy [[hidden sector]] particles which only interact with ordinary matter via gravity. These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.<ref name="bertone merritt" />

=== Direct particle detection ===
{{Further|Weakly interacting massive particle#Direct detection|Axion#Experimental searches}}
{{Main|Direct detection of dark matter}}

Direct detection experiments aim to observe interactions between dark matter particles passing through the Earth and ordinary matter detector targets. For [[Weakly interacting massive particles]] (WIMPs), the primary signature is a low-energy recoil of nuclei (typically a few [[keV]]), which induces energy in the form of [[scintillation (physics)|scintillation]] light, [[ionization]], or [[phonon]]s (heat). For [[axion]]s, experiments typically search for the conversion of axions into photons within a strong magnetic field (the [[Primakoff effect]]).

To detect these rare events effectively, it is crucial to maintain an extremely low background, which is why such experiments typically operate deep underground where interference from [[cosmic ray]]s is minimized. Major underground laboratories hosting these experiments include [[SNOLAB]] (Canada), [[Gran Sasso National Laboratory|LNGS]] (Italy), [[China Jinping Underground Laboratory|CJPL]] (China), and the [[Sanford Underground Research Facility|SURF]] (USA).

==== WIMPs ====
[[File:Direct Detection Constraints.png |Plot showing the parameter space of dark matter particle mass and interaction cross section with nucleons. The LUX and SuperCDMS limits exclude the parameter space above the labelled curves. The CoGeNT and CRESST-II regions indicate regions which were previously thought to correspond to dark matter signals, but which were later explained with mundane sources. The DAMA and CDMS-Si data remain unexplained, and these regions indicate the preferred parameter space if these anomalies are due to dark matter.|thumb]]

WIMP searches mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors, operating at temperatures below 100&nbsp;mK, detect the heat produced when a particle hits an atom in a crystal absorber such as [[germanium]]. Experiments using this technology include [[Cryogenic Dark Matter Search|SuperCDMS]] and [[EDELWEISS]].

[[Noble gas|Noble liquid]] detectors detect [[scintillation (physics)|scintillation]] and ionization produced by a particle collision in liquid [[xenon]] or [[argon]]. This technology has led the field in sensitivity for the last decade. Major current experiments include [[LZ experiment|LZ]] (at SURF), [[XENON|XENONnT]] (at LNGS), and [[PandaX|PandaX-4T]] (at CJPL), with future argon-based projects like [[DarkSide (dark matter experiment)|DarkSide-20k]] in development.

As of late 2025, there has been no confirmed detection of dark matter from these standard WIMP searches. Instead, experiments have placed strong upper limits on the particle's interaction cross-section with nucleons.<ref name="Akerib2017" /><ref name="Aprile2018" /> In late 2025, the [[LZ experiment]] reported the exclusion of WIMP cross-sections above 9 GeV/c<sup>2</sup> and the first detection of [[boron-8]] solar [[neutrino]]s via [[coherent elastic neutrino-nucleus scattering]] in a dark matter detector. This was the first experimental entry into the "neutrino fog", an irreducible background of neutrino interactions that mimics dark matter signals and complicates future WIMP searches.<ref name="LZ2025" />

==== Axions ====
As WIMP parameter space has become increasingly constrained, focus has also shifted toward [[axion]] searches. These experiments, such as the [[Axion Dark Matter Experiment]], typically use resonant microwave cavities rather than nuclear recoil targets. By the early 2020s, ADMX had achieved sensitivity to the plausible DFSZ axion model in the micro-electronvolt range.<ref name="Braine2020" />

==== Annual modulation and directionality ====
Despite the null results from major noble liquid and cryogenic experiments, the [[DAMA/NaI]] and [[DAMA/LIBRA]] collaborations have famously observed an annual modulation in their event rate,<ref>{{cite journal |last1=Bernabei |first1=R. |display-authors=et al. |year=2008 |title=First results from DAMA/LIBRA and the combined results with DAMA/NaI |journal=Eur. Phys. J. C |volume=56 |issue=3 |pages=333–355 |arxiv=0804.2741 |bibcode=2008EPJC...56..333B |doi=10.1140/epjc/s10052-008-0662-y |doi-access=free|s2cid=14354488}}</ref> which they claim is due to the Earth's motion through the [[dark matter halo]]. This claim remains in tension with the negative results from the more sensitive experiments (LZ, XENON, SuperCDMS) described above.

A special case of direct detection involves directional sensitivity, which attempts to correlate WIMP signals with the direction of the Solar System's motion towards [[Cygnus (constellation)|Cygnus]].<ref name="apssyn">{{cite news |last=Stonebraker |first=Alan |title=Synopsis: Dark-Matter Wind Sways through the Seasons |newspaper=Physics – Synopses |publisher=[[American Physical Society]] |date=3 January 2014 |doi=10.1103/PhysRevLett.112.011301 }}</ref> Directional experiments using low-pressure [[time projection chamber]]s include [[Dark Matter Time Projection Chamber|DMTPC]], [[Directional Recoil Identification From Tracks|DRIFT]], CYGNUS, and MIMAC.

=== Indirect particle detection ===
{{main|Indirect detection of dark matter}}
[[File:Collage of six cluster collisions with dark matter maps.jpg|thumb|upright=2|Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.<ref>{{cite web |title=Dark matter even darker than once thought |url=http://www.spacetelescope.org/news/heic1506/ |access-date=16 June 2015 |website=Space Telescope Science Institute |archive-date=12 June 2015 |archive-url=https://web.archive.org/web/20150612191132/http://www.spacetelescope.org/news/heic1506/ |url-status=live }}</ref>]]
[[File:Turning Black Holes into Dark Matter Labs.webm|thumb|Video about the potential [[Gamma-ray astronomy|gamma-ray detection]] of dark matter [[annihilation]] around [[supermassive black hole]]s. ''(Duration 0:03:13, also see file description.)'']]
Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the [[Galactic Center|centre of the Milky Way]]) two dark matter particles could [[Annihilation|annihilate]] to produce [[gamma ray]]s or Standard Model particle–antiparticle pairs.<ref name="Bertone2010">{{cite book |first=Gianfranco |last=Bertone |title=Particle Dark Matter: Observations, Models and Searches |chapter-url=https://books.google.com/books?id=JkUgAwAAQBAJ&pg=PA83 |year=2010 |publisher=Cambridge University Press |pages=83–104 |chapter=Dark Matter at the Centers of Galaxies |arxiv=1001.3706 |isbn=978-0-521-76368-4 |bibcode=2010arXiv1001.3706M}}</ref> Alternatively, if a dark matter particle is unstable, it could decay into Standard Model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, [[antiproton]]s or [[positron]]s emanating from high density regions in the Milky Way and other galaxies.<ref>{{cite journal |last1=Ellis |first1=J. |last2=Flores |first2=R. A. |last3=Freese |first3=K. |last4=Ritz |first4=S. |last5=Seckel |first5=D. |last6=Silk |first6=J. |year=1988 |title=Cosmic ray constraints on the annihilations of relic particles in the galactic halo |url=https://cds.cern.ch/record/190709/files/198809398.pdf |url-status=live |journal=Physics Letters B |volume=214 |issue=3 |pages=403–412 |bibcode=1988PhLB..214..403E |doi=10.1016/0370-2693(88)91385-8 |archive-url=https://web.archive.org/web/20180728133226/https://cds.cern.ch/record/190709/files/198809398.pdf |archive-date=2018-07-28}}</ref> A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.<ref name="bertone hooper silk" /><ref name="bertone merritt" />

A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy [[neutrino]]s.<ref>{{cite journal |doi=10.1016/0370-2693(86)90349-7 |author=Freese, K. |title=Can Scalar Neutrinos or Massive Dirac Neutrinos be the Missing Mass? |journal=Physics Letters B |volume=167 |issue=3 |pages=295–300 |date=1986 |bibcode=1986PhLB..167..295F}}</ref> Such a signal would be strong indirect proof of WIMP dark matter.<ref name="bertone hooper silk" /> High-energy neutrino telescopes such as [[Antarctic Muon And Neutrino Detector Array|AMANDA]], [[IceCube]] and [[ANTARES (telescope)|ANTARES]] are searching for this signal.<ref name=Randall_2015>
{{cite book
 |first=Lisa |last=Randall
 |year=2015
 |title=Dark Matter and the Dinosaurs: The astounding interconnectedness of the Universe
 |publisher=Ecco / HarperCollins Publishers
 |location=New York, NY
 |isbn=978-0-06-232847-2
}}</ref>{{rp|298}} Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow:

* The [[Energetic Gamma Ray Experiment Telescope]] observed more gamma rays in 2008 than expected from the [[Milky Way]], but scientists concluded this was most likely due to incorrect estimation of the telescope's sensitivity.<ref>{{cite journal |last1=Stecker |first1=F. W. |last2=Hunter |first2=S. |last3=Kniffen |first3=D. |year=2008 |title=The likely cause of the EGRET GeV anomaly and its implications |journal=Astroparticle Physics |volume=29 |issue=1 |pages=25–29 |arxiv=0705.4311 |bibcode=2008APh....29...25S |doi=10.1016/j.astropartphys.2007.11.002 |s2cid=15107441}}</ref>

* The [[Fermi Gamma-ray Space Telescope]] is searching for similar gamma rays.<ref>{{cite journal |title=The large area telescope on the Fermi Gamma-ray Space Telescope Mission |journal=Astrophysical Journal |volume=697 |issue=2 |year=2009 |pages=1071–1102 |doi=10.1088/0004-637X/697/2/1071 |arxiv=0902.1089 |first1=W.B. |last1=Atwood |last2=Abdo |first2=A.A. |last3=Ackermann |first3=M. |last4=Althouse |first4=W. |last5=Anderson |first5=B. |last6=Axelsson |first6=M. |last7=Baldini |first7=L. |last8=Ballet |first8=J. |last9=Band |first9=D.L. |display-authors=6 |bibcode=2009ApJ...697.1071A|s2cid=26361978 }}</ref> In 2009, an as yet unexplained surplus of gamma rays from the Milky Way's galactic center was found in Fermi data. This [[Galactic Center GeV excess]] might be due to dark matter annihilation or to a population of pulsars.<ref>{{cite web |date=2019-11-12 |title=Physicists revive hunt for dark matter in the heart of the Milky Way |url=https://www.science.org/content/article/physicists-revive-hunt-dark-matter-heart-milky-way |access-date=2023-05-09 |website=www.science.org |language=en}}</ref> In April 2012, an analysis of previously available data from Fermi's [[Fermi LAT|Large Area Telescope]] instrument produced statistical evidence of a 130&nbsp;GeV signal in the gamma radiation coming from the center of the Milky Way.<ref>{{cite journal |doi=10.1088/1475-7516/2012/08/007 |last=Weniger |first=Christoph |title=A tentative gamma-ray line from dark matter annihilation at the Fermi Large Area Telescope |journal=Journal of Cosmology and Astroparticle Physics |issue=8 |date=2012 |arxiv=1204.2797 |volume=2012 |page=7 |bibcode=2012JCAP...08..007W|s2cid=119229841 }}</ref> WIMP annihilation was seen as the most probable explanation.<ref>{{cite web |url=http://physicsworld.com/cws/article/news/2012/apr/24/gamma-rays-hint-at-dark-matter |title=Gamma rays hint at dark matter |last1=Cartlidge |first1=Edwin |date=24 April 2012 |publisher=Institute of Physics |access-date=23 April 2013}}</ref>

* At higher energies, [[IACT|ground-based gamma-ray telescopes]] have set limits on the annihilation of dark matter in [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]]<ref>{{Cite journal |last1=Albert |first1=J. |last2=Aliu |first2=E. |last3=Anderhub |first3=H. |last4=Antoranz |first4=P. |last5=Backes |first5=M. |last6=Baixeras |first6=C. |last7=Barrio |first7=J.A. |last8=Bartko |first8=H. |last9=Bastieri |first9=D. |last10=Becker |first10=J.K. |last11=Bednarek |first11=W. |last12=Berger |first12=K. |last13=Bigongiari |first13=C. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bordas |first16=P. |last17=Bosch-Ramon |first17=V. |last18=Bretz |first18=T. |last19=Britvitch |first19=I. |last20=Camara |first20=M. |last21=Carmona |first21=E. |last22=Chilingarian |first22=A. |last23=Commichau |first23=S. |last24=Contreras |first24=J.L. |last25=Cortina |first25=J. |last26=Costado |first26=M.T. |last27=Curtef |first27=V. |last28=Danielyan |first28=V. |last29=Dazzi |first29=F. |last30=De Angelis |first30=A. |display-authors=6 |title=Upper Limit for γ-Ray Emission above 140&nbsp;GeV from the Dwarf Spheroidal Galaxy Draco |doi=10.1086/529135 |journal=The Astrophysical Journal |volume=679 |issue=1 |pages=428–431 |year=2008 |arxiv=0711.2574 |bibcode=2008ApJ...679..428A|s2cid=15324383 }}</ref> and in clusters of galaxies.<ref>{{cite journal |last1=Aleksić |first1=J. |last2=Antonelli |first2=L.A. |last3=Antoranz |first3=P. |last4=Backes |first4=M. |last5=Baixeras |first5=C. |last6=Balestra |first6=S. |last7=Barrio |first7=J.A. |last8=Bastieri |first8=D. |last9=González |first9=J.B. |last10=Bednarek |first10=W. |last11=Berdyugin |first11=A. |last12=Berger |first12=K. |last13=Bernardini |first13=E. |last14=Biland |first14=A. |last15=Bock |first15=R.K. |last16=Bonnoli |first16=G. |last17=Bordas |first17=P. |last18=Tridon |first18=D.B. |last19=Bosch-Ramon |first19=V. |last20=Bose |first20=D. |last21=Braun |first21=I. |last22=Bretz |first22=T. |last23=Britzger |first23=D. |last24=Camara |first24=M. |last25=Carmona |first25=E. |last26=Carosi |first26=A. |last27=Colin |first27=P. |last28=Commichau |first28=S. |last29=Contreras |first29=J.L. |last30=Cortina |first30=J. |display-authors=6 |title=Magic Gamma-Ray Telescope observation of the Perseus Cluster of galaxies: Implications for cosmic rays, dark matter, and NGC&nbsp;1275 |doi=10.1088/0004-637X/710/1/634 |journal=The Astrophysical Journal |volume=710 |issue=1 |pages=634–647 |year=2010 |arxiv=0909.3267 |bibcode=2010ApJ...710..634A|s2cid=53120203 }}</ref>

* The [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]] experiment (launched in 2006) detected excess [[positron]]s. They could be from dark matter annihilation or from [[pulsar]]s. No excess [[antiproton]]s were observed.<ref>{{cite journal |last1=Adriani |first1=O. |last2=Barbarino |first2=G.C. |last3=Bazilevskaya |first3=G.A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E.A. |last7=Bonechi |first7=L. |last8=Bongi |first8=M. |last9=Bonvicini |first9=V. |last10=Bottai |doi=10.1038/nature07942 |first10=S. |last11=Bruno |first11=A. |last12=Cafagna |first12=F. |last13=Campana |first13=D. |last14=Carlson |first14=P. |last15=Casolino |first15=M. |last16=Castellini |first16=G. |last17=De Pascale |first17=M.P. |last18=De Rosa |first18=G. |last19=De Simone |first19=N. |last20=Di Felice |first20=V. |last21=Galper |first21=A.M. |last22=Grishantseva |first22=L. |last23=Hofverberg |first23=P. |last24=Koldashov |first24=S.V. |last25=Krutkov |first25=S.Y. |last26=Kvashnin |first26=A.N. |last27=Leonov |first27=A. |last28=Malvezzi |first28=V. |last29=Marcelli |first29=L. |last30=Menn |first30=W. |display-authors=6 |title=An anomalous positron abundance in cosmic rays with energies 1.5–100&nbsp;GeV |journal=Nature |volume=458 |issue=7238 |pages=607–609 |year=2009 |pmid=19340076 |arxiv=0810.4995 |bibcode=2009Natur.458..607A|s2cid=11675154 }}</ref>

* In 2013, results from the [[Alpha Magnetic Spectrometer]] on the [[International Space Station]] indicated excess high-energy [[cosmic ray]]s which could be due to dark matter annihilation.<ref name="APS-20130403">{{cite journal |title=First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350&nbsp;GeV |date=3 April 2013 |journal=[[Physical Review Letters]] |author=Aguilar, M. |collaboration=AMS Collaboration |doi=10.1103/PhysRevLett.110.141102 |bibcode=2013PhRvL.110n1102A |display-authors=etal |volume=110 |issue=14 |pmid=25166975 |article-number=141102|doi-access=free |hdl=1721.1/81241 |hdl-access=free }}</ref><ref name="AMS-20130403">{{cite web |title=First Result from the Alpha Magnetic Spectrometer Experiment |url=http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |date=3 April 2013 |author=AMS Collaboration |access-date=3 April 2013 |archive-url=https://web.archive.org/web/20130408185229/http://www.ams02.org/2013/04/first-results-from-the-alpha-magnetic-spectrometer-ams-experiment/ |archive-date=8 April 2013 }}</ref><ref name="AP-20130403">{{cite news |last1=Heilprin |first1=John |last2=Borenstein |first2=Seth |title=Scientists find hint of dark matter from cosmos |url=http://apnews.excite.com/article/20130403/DA5E6JAG3.html |date=3 April 2013 |agency=Associated Press |access-date=3 April 2013 |archive-date=10 May 2013 |archive-url=https://web.archive.org/web/20130510152050/http://apnews.excite.com/article/20130403/DA5E6JAG3.html }}</ref><ref name="BBC-20130403">{{cite news |last=Amos |first=Jonathan |title=Alpha Magnetic Spectrometer zeroes in on dark matter |url=https://www.bbc.co.uk/news/science-environment-22016504 |date=3 April 2013 |work=BBC |access-date=3 April 2013 |archive-date=12 August 2023 |archive-url=https://web.archive.org/web/20230812222642/https://www.bbc.com/news/science-environment-22016504 |url-status=live }}</ref><ref name="NASA-20130403">{{cite web |last1=Perrotto |first1=Trent J. |last2=Byerly |first2=Josh |title=NASA TV Briefing Discusses Alpha Magnetic Spectrometer Results |url=http://www.nasa.gov/home/hqnews/2013/apr/HQ_M13-054_AMS_Findings_Briefing.html |date=2 April 2013 |website=NASA |access-date=3 April 2013 |archive-date=5 January 2019 |archive-url=https://web.archive.org/web/20190105181536/https://www.nasa.gov/home/hqnews/2013/apr/HQ_M13-054_AMS_Findings_Briefing.html |url-status=live }}</ref><ref name="NYT-20130403">{{cite news |last=Overbye |first=Dennis |title=New Clues to the Mystery of Dark Matter |url=https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/04/04/science/space/new-clues-to-the-mystery-of-dark-matter.html |archive-date=2022-01-01 |url-access=limited |date=3 April 2013 |work=The New York Times |access-date=3 April 2013}}{{cbignore}}</ref>

The detection by [[LIGO]] in [[GW150914|September 2015]] of gravitational waves opens the possibility of observing dark matter in a new way, particularly if it is in the form of [[primordial black hole]]s.<ref>{{cite magazine |url=https://www.newscientist.com/article/2077800-what-will-gravitational-waves-tell-us-about-the-universe |title=Surfing gravity's waves |magazine=New Scientist |first=Joshua |last=Sokol |display-authors=etal |issue=3061 |date=20 February 2016}}</ref><ref>{{cite journal |last1=Bird |first1=Simeon |last2=Cholis |first2=Illian |year=2016 |title=Did LIGO detect dark matter? |journal=Physical Review Letters |volume=116 |issue=20 |article-number=201301 |doi=10.1103/PhysRevLett.116.201301 |pmid=27258861 |bibcode=2016PhRvL.116t1301B |arxiv=1603.00464|s2cid=23710177 }}</ref>

==== Astrophysical observations ====
[[File:Triangle of Everything.png|thumb|Logarithmic plot of size and mass of celestial objects from particles to galaxies]]
Beyond searching for annihilation products, astrophysicists are using celestial objects as natural detectors to constrain dark matter particle properties. 

* '''Stellar heating:''' If dark matter particles capture inside dense stars like [[neutron star]]s or [[white dwarf]]s, they can deposit kinetic energy during the capture process or through subsequent annihilation. This mechanism, known as "dark kinetic heating", would maintain the star at a temperature higher than expected for its age, potentially arresting its cooling indefinitely. The observation of old, "cold" neutron stars therefore places stringent limits on the scattering cross-section of dark matter particles with nucleons, as any significant interaction would have kept these stars hotter than observed.<ref name="Baryakhtar2017">{{cite journal |last1=Baryakhtar |first1=Masha |last2=Bramante |first2=Joseph |last3=Li |first3=Tim |last4=Linden |first4=Tim |last5=Raj |first5=N. |title=Dark Kinetic Heating of Neutron Stars and An Infrared Window On WIMPs, SIMPs, and Pure Higgsinos |journal=Physical Review Letters |volume=119 |issue=13 |article-number=131801 |year=2017 |arxiv=1704.01577 |doi=10.1103/PhysRevLett.119.131801 |pmid=29341667 |bibcode=2017PhRvL.119m1801B }}</ref><ref name="Raj2018">{{cite journal |last1=Raj |first1=Nirmal |last2=Tanedo |first2=Flip |last3=Yu |first3=Hai-Bo |title=Neutron stars at the dark matter direct detection frontier |journal=Physical Review D |volume=97 |issue=4 |article-number=043006 |year=2018 |arxiv=1707.09442 |doi=10.1103/PhysRevD.97.043006 |bibcode=2018PhRvD..97d3006R }}</ref>

* '''Stellar cooling:''' New light particles, such as axions, could be produced in the hot cores of stars and escape freely, carrying away energy. This additional energy loss channel would alter the evolution of stars, cooling them faster than standard models predict. Comparisons of observed [[red giant]] branch tips and [[white dwarf]] cooling curves with theoretical models have set some of the strongest constraints on the coupling of axions to electrons and photons.<ref name="Raffelt1990">{{cite book |last1=Raffelt |first1=Georg G. |title=Stars as laboratories for fundamental physics: the astrophysics of neutrinos, axions, and other weakly interacting particles |date=1996 |publisher=University of Chicago Press |location=Chicago |isbn=978-0-226-70272-8 |url=https://wwwth.mpp.mpg.de/members/raffelt/mypapers/Stars.pdf |access-date=25 January 2026}}</ref><ref>{{cite arXiv |last1=Fleury |first1=Leesa |last2=Obertas |first2=Alysa |last3=Richer |first3=Harvey |last4=Heyl |first4=Jeremy |date=26 November 2025 |title=Axion Constraints from White Dwarf Cooling in 47 Tucanae |eprint=2511.21676 |class=astro-ph.SR}}</ref>

* '''Black hole superradiance:''' Ultralight bosons, such as [[axion]]s or dark photons, can extract rotational energy from spinning black holes through a process called [[superradiance]]. If the boson's [[Compton wavelength]] is comparable to the black hole's event horizon size, the particles form a dense "boson cloud" around the black hole, rapidly slowing its spin on astrophysical timescales. The observation of rapidly spinning black holes in X-ray binaries or through gravitational waves excludes the existence of such particles in specific mass ranges, as their existence would have spun these black holes down long ago.<ref name="Cardoso2018">{{cite journal |last1=Cardoso |first1=Vitor |last2=Dias |first2=Óscar J. C. |last3=Hartnett |first3=Gavin S. |last4=Middleton |first4=Matthew |last5=Pani |first5=Paolo |last6=Santos |first6=Nuno M. |title=Constraining the mass of dark photons and axion-like particles through black-hole superradiance |journal=Journal of Cosmology and Astroparticle Physics |volume=2018 |issue=3 |page=043 |year=2018 |arxiv=1801.01420 |doi=10.1088/1475-7516/2018/03/043 |bibcode=2018JCAP...03..043C }}</ref><ref name="Witte2025">{{cite journal |last1=Witte |first1=Samuel J. |last2=Gou |first2=Lijun |last3=Brito |first3=Richard |title=Stepping up superradiance constraints on axions |journal=Physical Review D |volume=111 |issue=8 |article-number=083044 |year=2025 |arxiv=2412.03655 |doi=10.1103/PhysRevD.111.083044 |bibcode=2025PhRvD.111h3044W }}</ref>

=== Collider searches ===
[[File:EFTSimplifiedModels.svg|thumb|Schematic illustration of Dark Matter (DM) interactions and their corresponding experimental detection techniques, with time flowing from left to right. Fig. (a) shows DM annihilation to Standard Model (SM) particles, as sought by Indirect Detection (ID) experiments. Fig. (b) shows DM -> SM particle scattering, targeted by Direct Detection (DD) experiments. Fig. (c) shows the production of DM particles from the annihilation of SM particles at colliders. Fig. (d) shows again the pair production of DM at colliders as in (c), but in this case the interaction occurs through a mediator particle between DM and SM particles.]]
An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the [[Large Hadron Collider]] (LHC) may be able to detect dark matter particles produced in collisions of the LHC [[proton]] beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as large amounts of missing energy and momentum that escape the detectors, provided other non-negligible collision products are detected.<ref name="kane watson">{{cite journal |author1=Kane, G. |author2=Watson, S. |title=Dark Matter and LHC: what is the Connection? |journal=Modern Physics Letters A |year=2008 |volume=23 |pages=2103–2123 |doi=10.1142/S0217732308028314 |bibcode=2008MPLA...23.2103K |arxiv=0807.2244 |issue=26|s2cid=119286980 }}</ref>

==== Constraints on supersymmetry ====
For decades, the leading candidate for dark matter was the lightest [[neutralino]] predicted by [[supersymmetry]]. However, extensive searches through the conclusion of the LHC's run 3 (2022–2025) operations have failed to detect the superpartners (such as [[squark]]s and [[gluino]]s) predicted by supersymmetry models.<ref name="LHCRun3End">{{cite web |title=Accelerator Report: The final countdown to the end of Run 3 |url=https://home.cern/news/news/accelerators/accelerator-report-final-countdown |publisher=CERN |date=28 November 2025 |access-date=14 January 2026}}</ref> By late 2025, the [[ATLAS experiment|ATLAS]] and [[CMS experiment|CMS]] collaborations had pushed exclusion limits for gluinos beyond 2.4&nbsp;TeV, and limits for [[chargino]]s and neutralinos ("electroweak-inos") beyond 1&nbsp;TeV in many scenarios.<ref name="SUSYLimits2025">{{cite journal |last=Kwon |first=O. |title=The LHC has ruled out supersymmetry – really? |journal=Nuclear Physics B |date=15 May 2025 |volume=1018 |article-number=117012 |doi=10.1016/j.nuclphysb.2025.117012 |arxiv=2505.11251 |bibcode=2025NuPhB101817012C |url=https://arxiv.org/html/2505.11251v1}}</ref> This persistent absence has ruled out the most favored parameter space for WIMPs, forcing theorists to consider more complex and fine-tuned models such as "split supersymmetry", or to abandon supersymmetry candidates entirely.<ref name="SUSYLimits2025" />

==== Shift to dark sectors and exotic signatures ====
In response to these null results, experimental focus has shifted toward "dark sector" theories and more exotic signatures that might have evaded earlier experiments.<ref name="CMSDarkSector2024">{{cite web |title=CMS maps out Dark Matter searches |url=https://www.qu.uni-hamburg.de/research/highlights/24-06-06-cms-dm-searches-review.html |publisher=University of Hamburg |date=6 June 2024 |access-date=20 January 2026 |archive-date=3 March 2026 |archive-url=https://web.archive.org/web/20260303023600/https://www.qu.uni-hamburg.de/research/highlights/24-06-06-cms-dm-searches-review.html |url-status=live }}</ref> Recent analyses from 2024 and 2025 have targeted signatures that do not fit the expected missing energy profile:
* '''Long-lived particles:''' These are particles that travel centimeters or meters through the detector before decaying, creating "displaced vertices" or "disappearing tracks." New triggers implemented in Run 3 specifically targeted these events, particularly looking for long-lived charginos that decay into invisible dark matter and very soft [[pion]]s.<ref name="ATLASRun3LLP">{{cite web |title=ATLAS probes uncharted territory with LHC Run 3 data |url=https://home.cern/news/news/physics/atlas-probes-uncharted-territory-lhc-run-3-data |publisher=CERN |date=26 July 2024 |access-date=18 January 2026 |archive-date=13 January 2026 |archive-url=https://web.archive.org/web/20260113023145/https://home.cern/news/news/physics/atlas-probes-uncharted-territory-lhc-run-3-data |url-status=live }}</ref>
* '''Dark jets and semi-visible jets:''' Signatures where dark matter is produced alongside visible matter in complex showers, which look different from standard quark-gluon [[Jet (particle physics)|jets]]. In 2025, ATLAS released results on "emerging jets" that appear mid-flight within the detector, setting the first exclusion limits on dark hadrons in that channel.<ref name="ATLASDarkJets">{{cite web |title=Shedding light with jets from the dark side |url=https://atlas.cern/Updates/Briefing/Shedding-Light-Dark-Sector |publisher=ATLAS Experiment at CERN |date=14 May 2025 |access-date=15 January 2026}}</ref>
* '''Dark photons:''' Lighter mediators that could bridge the Standard Model and the dark sector. Experiments like the FASER experiment and dedicated low-mass triggers at CMS have searched for these in the 2–8&nbsp;GeV mass range, constraining the mixing parameters between dark and ordinary photons.<ref name="CMSDarkPhoton2025">{{cite conference |title=Search for Massive Dark Photons with the CMS Experiment |book-title=SMT 2025: Searches and QCD Studies at Colliders |date=April 2025 |publisher=American Physical Society |url=https://archive.aps.org/smt/2025/apr-r13/1/}}</ref>

While the LHC has not yet produced direct evidence of dark matter, the constraints established by the ATLAS and CMS collaborations have been crucial in narrowing their parameter spaces, closing the door on many WIMP models and redirecting future searches toward lighter, more elusive candidates or multi-TeV scales accessible only by future colliders like the [[Future Circular Collider]].<ref name="ATLASRun3Summary">{{cite conference |title=Searches for dark matter using LHC Run-2 and Run-3 data recorded by the ATLAS experiment |conference=Cosmology 2025 @ Elba Island |date=9 September 2025 |url=https://indico.global/event/7910/contributions/135371/contribution.pdf}}</ref>

== Alternative hypotheses ==
=== Modified gravity ===
{{Further|Alternatives to general relativity}}

If dark matter is not an undiscovered particle, then the next possibility is that general relativity, the theory underpinning modern cosmology, is incorrect. General relativity is well-tested on Solar System scales, but its validity on galactic or cosmological scales has not been well proven.<ref>{{cite book | author = Peebles, P. J. E.| date= December 2004| arxiv= astro-ph/0410284|bibcode = 2005grg..conf..106P |doi = 10.1142/9789812701688_0010 | isbn = 978-981-256-424-5 | pages = 106–117 | chapter= Probing General Relativity on the Scales of Cosmology| title= General Relativity and Gravitation| s2cid= 1700265}}</ref> A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are [[modified Newtonian dynamics]] (MOND) and its relativistic generalization [[tensor–vector–scalar gravity]] (TeVeS),<ref>For a review, see: {{cite journal |title=The failures of the Standard Model of Cosmology require a new paradigm |author1=Kroupa, Pavel |display-authors=etal |journal=International Journal of Modern Physics D |date=December 2012 |volume=21 |issue=4 |page=1230003 |doi=10.1142/S0218271812300030 |arxiv=1301.3907 |bibcode=2012IJMPD..2130003K|s2cid=118461811 }}</ref> [[f(R) gravity]],<ref>For a review, see: {{cite journal |title=The dark matter problem from f(R) gravity viewpoint |author=Salvatore Capozziello |author2=Mariafelicia De Laurentis |journal=Annalen der Physik |date=October 2012 |volume=524 |issue=9–10 |page=545 |doi=10.1002/andp.201200109 |bibcode=2012AnP...524..545C |doi-access=free }}</ref> [[negative mass]], [[dark fluid]],<ref>{{cite web|url= https://www.ox.ac.uk/news/2018-12-05-bringing-balance-universe |title=Bringing balance to the Universe |date=5 December 2018 |publisher=University of Oxford}}</ref><ref>{{cite web |url= https://phys.org/news/2018-12-universe-theory-percent-cosmos.html |title= Bringing balance to the universe: New theory could explain missing 95 percent of the cosmos |publisher= Phys.Org |access-date= 1 February 2019 |archive-date= 2 February 2019 |archive-url= https://web.archive.org/web/20190202095205/https://phys.org/news/2018-12-universe-theory-percent-cosmos.html |url-status= live }}</ref><ref name="Farnes">{{cite journal |last=Farnes |first=J. S. |year=2018 |title=A Unifying Theory of Dark Energy and Dark Matter: Negative Masses and Matter Creation within a Modified ΛCDM Framework |journal=Astronomy & Astrophysics |volume=620 |page=A92 |arxiv=1712.07962 |bibcode=2018A&A...620A..92F |doi=10.1051/0004-6361/201832898 |s2cid=53600834}}</ref> [[entropic gravity]],<ref name="physorgnewtheory">{{cite news |title=New theory of gravity might explain dark matter |url=https://phys.org/news/2016-11-theory-gravity-dark.html |website=phys.org |date=November 2016}}</ref> [[conformal gravity]], and [[massive gravity]]. [[Alternatives to general relativity|Alternative theories]] abound.<ref>{{cite journal |title=Alternatives to dark matter and dark energy |first=Phillip D. |last=Mannheim |journal=Progress in Particle and Nuclear Physics |volume=56 |issue=2 |pages=340–445 |doi=10.1016/j.ppnp.2005.08.001 |arxiv=astro-ph/0505266 |date=April 2006 |bibcode=2006PrPNP..56..340M |s2cid=14024934 }}</ref><ref>{{cite journal |title=Beyond the Cosmological Standard Model |first1=Austin |last1=Joyce |display-authors=etal |journal=Physics Reports |date=March 2015 |volume=568 |pages=1–98 |doi=10.1016/j.physrep.2014.12.002 |arxiv=1407.0059 |bibcode=2015PhR...568....1J|s2cid=119187526 }}</ref>

A problem with modifying gravity is that observational evidence for dark matter – let alone general relativity – comes from so many independent approaches (see [[Dark matter#Observational evidence|§ Observational evidence]] above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity<ref>{{cite news |url=http://phys.org/news/2016-12-verlinde-theory-gravity.html |title=Verlinde's new theory of gravity passes first test |date=16 December 2016 |archive-date=3 January 2022 |access-date=20 February 2017 |archive-url=https://web.archive.org/web/20220103063834/https://phys.org/news/2016-12-verlinde-theory-gravity.html |url-status=live }}</ref><ref>{{cite journal |title=First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements |first1=Margot M. |last1=Brouwer |display-authors=etal |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=466 |issue=3 |date=April 2017 |doi=10.1093/mnras/stw3192 |arxiv=1612.03034 |pages=2547–2559 |doi-access=free |bibcode=2017MNRAS.466.2547B|s2cid=18916375 }}</ref><ref>{{cite web |last1=Fullname&#125 |first1=#Author |url=https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/ |title=First test of rival to Einstein's gravity kills off dark matter |work=New Scientist |date=15 December 2016 |access-date=20 February 2017 |archive-date=5 January 2022 |archive-url=https://web.archive.org/web/20220105220635/https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/ |url-status=live }}</ref> and a 2020 measurement of a unique MOND effect.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2020/12/201216155158.htm|title=Unique prediction of 'modified gravity' challenges dark matter|publisher=ScienceDaily|date=16 December 2020|access-date=14 January 2021}}</ref><ref>{{cite journal|title=Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies|first1=Kyu-Hyun|last1=Chae|display-authors=et al|journal=[[Astrophysical Journal]]|volume=904|date=20 November 2020|issue=1|page=51|doi=10.3847/1538-4357/abbb96|arxiv=2009.11525|bibcode=2020ApJ...904...51C|s2cid=221879077 |doi-access=free }}</ref> The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter present in the universe.<ref name="CarrollTrialogue">{{cite web |author=Carroll |first=Sean |author-link=Sean M. Carroll |date=9 May 2012 |title=Dark matter vs. modified gravity: A trialogue |url=http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ |access-date=14 February 2017 |archive-date=14 February 2017 |archive-url=https://web.archive.org/web/20170214181733/http://www.preposterousuniverse.com/blog/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ |url-status=live }}</ref>

=== Non-mainstream and less established particle, field, and structure theories ===
While WIMPs, axions, and primordial black holes remain the primary candidates for dark matter, numerous other theories have been proposed to address specific observational anomalies or theoretical motivations. These alternative models often explore mass ranges and interaction strengths outside the standard parameter space, ranging from ultra-light scalar fields to massive composite states. Some hypotheses posit the existence of complex "dark sectors" with their own fundamental forces, while others suggest that dark matter may be unstable, dynamical, or composed of mirror particles. The following list encompasses these less established but theoretically motivated candidates and frameworks.

* {{annotated link|Chameleon particle}}
* {{annotated link|Dark galaxy}}
* {{annotated link|Dark radiation}}
* [[Density wave theory]] – A theory in which waves of compressed gas, which move slower than the galaxy, maintain galaxies' structure
* [[Dynamical dark matter]]<ref name=DienesThomas2012>{{cite journal |last1=Dienes |first1=Keith R. |last2=Thomas |first2=Brooks |date=2012-04-24 |title=Dynamical dark matter. I. Theoretical overview |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.083523 |journal=Physical Review D |volume=85 |issue=8 |article-number=083523 |doi=10.1103/PhysRevD.85.083523 |arxiv=1106.4546 |bibcode=2012PhRvD..85h3523D |archive-date=9 September 2024 |access-date=9 September 2024 |archive-url=https://web.archive.org/web/20240909150828/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.083523 |url-status=live }}</ref><ref>{{cite journal |last1=Dienes |first1=Keith R. |last2=Thomas |first2=Brooks |title=Dynamical dark matter. II. An explicit model |journal=Physical Review D |date=24 April 2012 |volume=85 |issue=8 |article-number=083524 |doi=10.1103/PhysRevD.85.083524 |arxiv=1107.0721 |bibcode=2012PhRvD..85h3524D |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.083524 |issn=1550-7998 |archive-date=27 February 2026 |access-date=23 January 2026 |archive-url=https://web.archive.org/web/20260227125516/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.85.083524 |url-status=live }}</ref>
* {{annotated link|Exotic matter}}
* {{annotated link|Feebly interacting particle}}s
* {{annotated link|Light dark matter}}
* {{annotated link|Mirror matter}}
* {{annotated link|Neutralino#Relationship to dark matter|Neutralino}}
* {{annotated link|Scalar field dark matter}}
* {{annotated link|Strongly interacting massive particle|abbreviation=SIMP}}
* [[WISP (particle physics)|Weakly interacting slim particle]] (WISP){{snd}}Low-mass counterpart to WIMP

== In popular culture ==
Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,<ref>
{{cite journal
 |last=Cramer |first=John G.
 |date=1 July 2003
 |title=LSST – the dark matter telescope
 |journal=[[Analog Science Fiction and Fact]]
 |volume=123 |issue=7/8 |page=96
 |issn=1059-2113 |id={{ProQuest|215342129}}
}} (Registration required)
</ref> and dark matter itself has been referred to as "the stuff of science fiction".<ref>
{{cite news
 |last=Ahern |first=James
 |date=16 February 2003
 |title=Space travel: Outdated goal
 |page=O&nbsp;02
 |work=[[The Record (Bergen County)|The Record]]
 |id={{ProQuest|425551312}}
}} (Registration required)
</ref>

Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example:
* Dark matter serves as a plot device in the 1995 ''[[The X-Files|X-Files]]'' episode "[[Soft Light (The X-Files)|Soft Light]]".<ref>
{{cite magazine
 |first=Grace |last=Halden
 |date=Spring 2015
 |title=Incandescent: Light bulbs and conspiracies
 |magazine=[[Dandelion (magazine)|Dandelion]]: Postgraduate Arts Journal and Research Network
 |volume=5 |number=2
 |doi=10.16995/ddl.318 |doi-access=free
}}
</ref> 
* A dark-matter-inspired substance known as ''"Dust"'' features prominently in [[Philip Pullman]]'s ''[[His Dark Materials]]'' trilogy.<ref>
{{cite book
 |first1=Mary |last1=Gribbin
 |first2=John |last2=Gribbin
 |year=2007
 |title=The Science of Philip Pullman's His Dark Materials
 |publisher=Random House Children's Books
 |isbn=978-0-375-83146-1
 |pages=15–30
}}
</ref>
* Beings made of dark matter are antagonists in [[Stephen Baxter (author)|Stephen Baxter]]'s ''[[Xeelee Sequence]]''.<ref>{{cite journal
 |first=Andrew
 |last=Fraknoi
 |year=2019
 |title=Science fiction for scientists
 |journal=[[Nature Physics]]
 |volume=12
 |issue=9
 |pages=819–820
 |doi=10.1038/nphys3873
 |s2cid=125376175
 |url=https://www.nature.com/articles/nphys3873
 |url-access=subscription
 |archive-date=11 December 2022
 |access-date=11 December 2022
 |archive-url=https://web.archive.org/web/20221211184438/https://www.nature.com/articles/nphys3873
 |url-status=live
 }}</ref>

More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.<ref>
{{cite web
 |first=Adam |last=Frank |author-link=Adam Frank
 |date=9 February 2017
 |title=Dark matter is in our DNA
 |website=[[Nautilus Quarterly]]
 |url=https://nautil.us/dark-matter-is-in-our-dna-2-236435/
 |access-date=2022-12-11
}}
</ref>
<!-- Please do not add trivia about dark matter in popular culture to this section. Examples require secondary sources that provide context and indicate how those examples are significant. -->

== Gallery ==
{{Gallery
|File:COSMOSDMmap2007.jpg|DM map by the [[Cosmic Evolution Survey]] (COSMOS) using the [[Hubble Space Telescope]] (2007)<ref>{{cite web|title=First 3D map of the Universe's dark matter scaffolding|url=https://www.esa.int/Science_Exploration/Space_Science/First_3D_map_of_the_Universe_s_dark_matter_scaffolding|access-date=2021-11-23|website=www.esa.int|language=en|archive-date=23 November 2021|archive-url=https://web.archive.org/web/20211123162856/https://www.esa.int/Science_Exploration/Space_Science/First_3D_map_of_the_Universe_s_dark_matter_scaffolding|url-status=live}}</ref><ref>{{cite journal|last1=Massey|first1=Richard|last2=Rhodes|first2=Jason|last3=Ellis|first3=Richard|last4=Scoville|first4=Nick|last5=Leauthaud|first5=Alexie|last6=Finoguenov|first6=Alexis|last7=Capak|first7=Peter|last8=Bacon|first8=David|last9=Aussel|first9=Hervé|last10=Kneib|first10=Jean-Paul|last11=Koekemoer|first11=Anton|date=January 2007|title=Dark matter maps reveal cosmic scaffolding|url=https://www.nature.com/articles/nature05497|journal=Nature|language=en|volume=445|issue=7125|pages=286–290|doi=10.1038/nature05497|pmid=17206154|arxiv=astro-ph/0701594|bibcode=2007Natur.445..286M|s2cid=4429955|issn=1476-4687|archive-date=23 November 2021|access-date=27 November 2021|archive-url=https://web.archive.org/web/20211123233435/https://www.nature.com/articles/nature05497|url-status=live}}</ref>

|File:CFHTLenSDMmap2012.jpg|DM map by the CFHT Lensing Survey (CFHTLenS) using the [[Canada–France–Hawaii Telescope]] (2012)<ref>{{cite web|title=News CFHT - Astronomers reach new frontiers of dark matter|url=https://www.cfht.hawaii.edu/en/news/CFHTLens/|access-date=2021-11-26|website=www.cfht.hawaii.edu|archive-date=7 April 2022|archive-url=https://web.archive.org/web/20220407210206/https://www.cfht.hawaii.edu/en/news/CFHTLens/|url-status=live}}</ref><ref>{{cite journal|last1=Heymans|first1=Catherine|last2=Van Waerbeke|first2=Ludovic|last3=Miller|first3=Lance|last4=Erben|first4=Thomas|last5=Hildebrandt|first5=Hendrik|last6=Hoekstra|first6=Henk|last7=Kitching|first7=Thomas D.|last8=Mellier|first8=Yannick|last9=Simon|first9=Patrick|last10=Bonnett|first10=Christopher|last11=Coupon|first11=Jean|date=2012-11-21|title=CFHTLenS: the Canada–France–Hawaii Telescope Lensing Survey: CFHTLenS|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=427|issue=1|pages=146–166|arxiv=1210.0032|doi=10.1111/j.1365-2966.2012.21952.x|doi-access=free |s2cid=24731530}}</ref> (COSMOS map at the center)

|File:KiDSDMmap2015.gif|DM map by the Kilo-Degree Survey (KiDS) using the [[VLT Survey Telescope]] (2015)<ref>{{cite web|title=KiDS|url=https://kids.strw.leidenuniv.nl/pr_july2015.php|access-date=2021-11-27|website=kids.strw.leidenuniv.nl|archive-date=27 November 2021|archive-url=https://web.archive.org/web/20211127194829/https://kids.strw.leidenuniv.nl/pr_july2015.php|url-status=live}}</ref><ref>{{cite journal|last1=Kuijken|first1=Konrad|last2=Heymans|first2=Catherine|last3=Hildebrandt|first3=Hendrik|last4=Nakajima|first4=Reiko|last5=Erben|first5=Thomas|last6=Jong|first6=Jelte T. A.|last7=Viola|first7=Massimo|last8=Choi|first8=Ami|last9=Hoekstra|first9=Henk|last10=Miller|first10=Lance|last11=van Uitert|first11=Edo|date=10 October 2015|title=Gravitational lensing analysis of the Kilo-Degree Survey|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=454|issue=4|pages=3500–3532|arxiv=1507.00738|doi=10.1093/mnras/stv2140|doi-access=free |issn=0035-8711}}</ref>

|File:HSCSDMmap2018.gif|DM map by the Hyper Suprime-Cam Survey (HSCS) using the [[Subaru Telescope]] (2018)<ref>{{cite web|last=University|first=Carnegie Mellon|date=26 September 2018|title=Hyper Suprime-Cam Survey Maps Dark Matter in the Universe - News - Carnegie Mellon University|url=http://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html|url-status=live|website=www.cmu.edu|language=en|archive-url=https://web.archive.org/web/20200907194216/https://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html |archive-date=7 September 2020 }}</ref><ref>{{cite journal|last1=Hikage|first1=Chiaki|last2=Oguri|first2=Masamune|last3=Hamana|first3=Takashi|last4=More|first4=Surhud|last5=Mandelbaum|first5=Rachel|last6=Takada|first6=Masahiro|last7=Köhlinger|first7=Fabian|last8=Miyatake|first8=Hironao|last9=Nishizawa|first9=Atsushi J|last10=Aihara|first10=Hiroaki|last11=Armstrong|first11=Robert|date=2019-04-01|title=Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-year data|url=https://academic.oup.com/pasj/article/doi/10.1093/pasj/psz010/5370019|journal=Publications of the Astronomical Society of Japan|language=en|volume=71|issue=2|page=43|arxiv=1809.09148|doi=10.1093/pasj/psz010|issn=0004-6264|archive-date=25 November 2021|access-date=27 November 2021|archive-url=https://web.archive.org/web/20211125194844/https://academic.oup.com/pasj/article/doi/10.1093/pasj/psz010/5370019|url-status=live}}</ref>

|File:DESDMmap2021.png|DM map by the [[Dark Energy Survey]] (DES) using the [[Víctor M. Blanco Telescope]] (2021)<ref>{{cite journal|last1=Jeffrey|first1=N|last2=Gatti|first2=M|last3=Chang|first3=C|last4=Whiteway|first4=L|last5=Demirbozan|first5=U|last6=Kovacs|first6=A|last7=Pollina|first7=G|last8=Bacon|first8=D|last9=Hamaus|first9=N|last10=Kacprzak|first10=T|last11=Lahav|first11=O|date=2021-06-25|title=Dark Energy Survey Year 3 results: Curved-sky weak lensing mass map reconstruction|url=https://academic.oup.com/mnras/article/505/3/4626/6287258|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=505|issue=3|pages=4626–4645|arxiv=2105.13539|doi=10.1093/mnras/stab1495|doi-access=free|issn=0035-8711|archive-date=17 December 2021|access-date=27 November 2021|archive-url=https://web.archive.org/web/20211217061541/https://academic.oup.com/mnras/article/505/3/4626/6287258|url-status=live}}</ref><ref>{{cite journal|last=Castelvecchi|first=Davide|date=2021-05-28|title=The most detailed 3D map of the Universe ever made|url=http://www.nature.com/articles/d41586-021-01466-1|journal=Nature|language=en|pages=d41586–021–01466-1|doi=10.1038/d41586-021-01466-1|pmid=34050347|s2cid=235242965|issn=0028-0836|url-access=subscription|archive-date=26 November 2021|access-date=27 November 2021|archive-url=https://web.archive.org/web/20211126161455/https://www.nature.com/articles/d41586-021-01466-1|url-status=live}}</ref>
}}

== See also ==
; Related theories
* {{annotated link|Dark energy}}
* {{annotated link|Unparticle physics}}
; Experiments
* {{annotated link|DEAP}}, a search apparatus
* {{annotated link|Dark Matter Particle Explorer|abbreviation=DAMPE}}
* {{annotated link|General antiparticle spectrometer}}
* {{annotated link|MultiDark}}, a research program
* {{annotated link|Illustris project}}, astrophysical simulations
; Other
* {{annotated link|Galactic Center GeV excess}}
* [[Luminiferous aether]] – A once theorized invisible and infinite material with no interaction with physical objects, used to explain how light could travel through a vacuum (now disproven)

== Notes ==
{{notelist|1}}

== References ==
{{reflist|25em}}

== Further reading ==
* {{cite book |last=Ferreras |first=Ignacio |title=Fundamentals of Dark Matter |year=2025 |publisher=UCL Press |isbn=978-1-80008-470-4 |url=https://uclpress.co.uk/book/fundamentals-of-dark-matter/}}
* {{cite book |last1=Freeman |first1=Ken |title=In Search of Dark Matter |last2=MacNamara |first2=Geoff |date=2006 |publisher=Springer/Praxis |isbn=978-0-387-27616-8 |series=Springer-Praxis Books in Popular Astronomy |location=Berlin, Springer, Chichester}}
* {{cite book |editor-last1=Kimball |editor-first1=Derek |editor-last2=Bibber |editor-first2=Karl |title=The Search for Ultralight Bosonic Dark Matter |year=2023 |url=https://link.springer.com/book/10.1007/978-3-030-95852-7 |isbn=978-3-030-95852-7 |publisher=[[Springer Nature]]|doi=10.1007/978-3-030-95852-7 |bibcode=2023subd.book.....K }}
* {{cite book |last=Sanders |first=Robert H. |title=The Dark Matter Problem: A historical perspective |date=2010 |publisher=Cambridge University Press |isbn=978-0-511-77357-0 |location=Cambridge, New York}}
* {{cite book |last1=Overduin |first1=James M. |title=Dark Sky, Dark Matter |last2=Wesson |first2=Paul S. |date=2003 |publisher=Institute of Physics |isbn=978-0-7503-0684-3 |series=Series in Astronomy and Astrophysics |location=Bristol}} 
* {{cite book |last=Bertone |first=Gianfranco |title=Particle Dark Matter: Observations, models and searches |date=2010 |publisher=Cambridge University Press |isbn=978-0-521-76368-4 |location=Cambridge}}
* {{cite book |last=Panek |first=Richard |title=The 4 Percent Universe: Dark matter, dark energy, and the race to discover the rest of reality |date=2011 |publisher=Houghton Mifflin Harcourt |isbn=978-0-618-98244-8 |location=Boston}}
* [[Rainer Weiss|Weiss, Rainer]], (July/August 2023) "The Dark Universe Comes into Focus" ''[[Scientific American]]'', vol. 329, no. 1, pp.&nbsp;7–8.

== External links ==
{{Commons category|Dark matter}}
* {{cite AV media |url=https://www.ias.edu/video/the-fifth-element |title=Lecture on dark matter |author=Tremaine, Scott |publisher=IAS |medium=Video }}
* {{cite web |last1=Gray |first1=Meghan |title=Dark Matter |url=http://www.sixtysymbols.com/videos/darkmatter.htm |website=Sixty Symbols |editor-link=Brady Haran |editor=Haran, Brady |publisher=[[University of Nottingham]] |author2=Merrifield, Mike |author3=Copeland, Ed |date=2010}}

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