Editing Cold dark matter (section)

==Composition==
Dark matter is detected through its gravitational interactions with ordinary matter and radiation. As such, it is very difficult to determine what the constituents of cold dark matter are. The candidates fall roughly into three categories:

* [[Axion]]s, very light particles with a specific type of self-interaction that makes them a suitable CDM candidate.<ref name="Axion, first CDM candidate">
{{cite news
 | author1 = Turner, M.
 | display-authors=etal
 | year = 2010
 | title = Axions 2010 Workshop
 | publisher = U. Florida
 | place = Gainesville, USA
}}{{full citation|date=August 2021}}
</ref><ref name="Axion Cosmology">
{{cite news
 | author1 = Sikivie, Pierre
 | display-authors=etal
 | year= 2008
 | title = Axion Cosmology
 | work = Lect. Notes Phys.
 | volume = 741
 | pages = 19–50
}}{{full citation|date=August 2021}}
</ref> Since the late 2010s, axions have become one of the most promising candidates for dark matter.<ref name="Chadha-Day et al">{{cite journal |title=Axion dark matter: What is it and why now?|author1=Francesca Chadha-Day|author2=John Ellis|author3=David J. E. Marsh|journal=Science Advances|volume=8|issue=8|doi=10.1126/sciadv.abj3618|pmid=35196098|date=23 February 2022|pages=eabj3618 |pmc=8865781 |arxiv=2105.01406 |bibcode=2022SciA....8J3618C }}</ref> Axions have the theoretical advantage that their existence solves the [[strong CP problem]] in [[quantum chromodynamics]], but axion particles have only been theorized and never detected. Axions are an example of a more general category of particle called a [[WISP (particle physics)|WISP]] ([[WISP (particle physics)|weakly interacting "slender" or "slim" particle]]), which are the low-mass counterparts of WIMPs. 

* [[Massive compact halo object]]s (MACHOs), large, condensed objects such as [[black hole]]s, [[neutron star]]s, [[white dwarf]]s, very faint [[star]]s, or non-luminous objects like [[planet]]s. The search for these objects consists of using [[gravitational lensing]] to detect the effects of these objects on background galaxies. Most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate.<ref name=Carr>
{{cite journal
 | last1=Carr | first1=B.J.
 | display-authors=etal
 | date=May 2010
 | title=New cosmological constraints on primordial black holes
 | journal=Physical Review D
 | volume=81 | issue=10 | page=104019
 | doi=10.1103/PhysRevD.81.104019 |arxiv=0912.5297
 |bibcode = 2010PhRvD..81j4019C |s2cid=118946242
}}
</ref><ref name=Peter>
{{cite arXiv
 |last=Peter |first=A.H.G.
 |year=2012
 |title=Dark matter: A brief review
 |eprint=1201.3942
 |class=astro-ph.CO
}}
</ref><ref name="bertone hooper silk">
{{cite journal
 |last1=Bertone | first1=Gianfranco
 | last2=Hooper | first2=Dan
 | last3=Silk   | first3=Joseph | author3-link=Joseph Silk
 | date=January 2005
 | title=Particle dark matter: evidence, candidates and constraints
 | doi=10.1016/j.physrep.2004.08.031
 | arxiv = hep-ph/0404175
 | journal=Physics Reports
 | volume=405 |issue=5–6 | pages=279–390
 | bibcode=2005PhR...405..279B | s2cid=118979310
}}
</ref><ref name=Garrett>
{{cite journal
 | last1 = Garrett | first1 = Katherine
 | last2 = Dūda    | first2 = Gintaras
 | year = 2011
 | title = Dark Matter: A Primer
 | journal = Advances in Astronomy
 | volume = 2011 | page = 968283
 | doi = 10.1155/2011/968283 |arxiv = 1006.2483
 |bibcode = 2011AdAst2011E...8G | s2cid = 119180701
 |quote=MACHOs can only account for a very small percentage of the nonluminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is {{'}}''no''{{'}}&nbsp;...
| doi-access = free
 }}
</ref><ref name=Bertone>
{{cite journal
 |last=Bertone |first=Gianfranco
 |date=18 November 2010 
 |title=The moment of truth for WIMP dark matter
 |journal=Nature
 |volume=468 |issue=7322
 |pages=389–393
|doi=10.1038/nature09509
 |pmid=21085174
 |arxiv=1011.3532
 |bibcode=2010Natur.468..389B
 |s2cid=4415912
 |url=https://www.zora.uzh.ch/id/eprint/41577/1/1011.3532v1.pdf
 }}
</ref><ref name=Olive>
{{cite journal
 | last1 = Olive | first1 = Keith A.
 | year = 2003
 | title = TASI lectures on dark matter
 | journal = Physics
 | volume = 54 | page = 21
 | bibcode = 2003astro.ph..1505O | arxiv = astro-ph/0301505
}}
</ref>

* [[Weakly interacting massive particles]] (WIMPs). There is no currently known particle with the required properties, but many extensions of the [[standard model of particle physics]] predict such particles. The search for WIMPs involves attempts at direct detection by highly sensitive detectors, as well as attempts at production of WIMPs by [[particle accelerator]]s. Historically, WIMPs were regarded as one of the most promising candidates for the composition of dark matter,<ref name=Peter/><ref name=Garrett/><ref name=Olive/> but since the late 2010s, WIMPs have been supplanted by axions with the non-detection of WIMPs in experiments.<ref name="Chadha-Day et al"/> The [[DAMA/NaI]] experiment and its successor [[DAMA/LIBRA]] have claimed to have directly detected dark matter particles passing through the Earth, but many scientists remain skeptical because no results from similar experiments seem compatible with the DAMA results.