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One candidate for dark matter is axions and there is tentative experimental evidence for their existence.

Axions are very light, most models weigh them in at tiny fractions of an eV. It seems like such a light particle could never slow down enough. A "heavy" 1eV axion cloud, even if cooled to the CMB temperature of 2.7K, has an RMS velocity of ~3-4% of the speed of light. This would rapidly disperse and could not clump inside of galaxies or clusters.

So how can such light particles cool off enough to slow down and condense?

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  • $\begingroup$ It should be noted that a followup experiment to the one described in the linked article seemed to contradict the results of the first experiment: arxiv.org/abs/2207.11330. This doesn't have to do with the question itself, but it's notable since experimental evidence for axions was brought up. $\endgroup$
    – Giorgos G
    Commented Jun 5 at 10:52

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Thermally produced light axions could not be cold dark matter for the reasons you identify.

The CDM axions are hypothesised to be produced by (Peccei-Quinn) symmetry breaking around the epoch of inflation. Since the particles are bosons, they then all sit/condense in the lowest energy state at the bottom of a potential well and have very little dispersion in speed. Since there a great number of these particles produced they can act like cold dark matter.

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  • $\begingroup$ Instead of "it's so cold that a BEC forms" you are saying "it's so dense that a BEC forms even though it isn't cold"? Makes me wonder if DM halos would have a "cut-off" beyond which the density drops too low to form a BEC and the particles disperse. $\endgroup$
    – Kevin Kostlan
    Commented Jun 6 at 16:26

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