  Axions differ from ordinary cold dark matter, such as WIMPs or sterile
neutrinos, because they form a Bose-Einstein condensate (BEC). As a result,
axions accreting onto a galactic halo fall in with net overall rotation. In
contrast, ordinary CDM accretes onto galactic halos with an irrotational
velocity field. The inner caustics are different in the two cases. It is shown
that if the dark matter is axions, the phase space structure of the halos of
isolated disk galaxies, such as the Milky Way, is precisely that of the caustic
ring model for which observational support exists. The other dark matter
candidates predict a far more chaotic phase space structure for galactic halos.
