  Dark matter axions form a rethermalizing Bose-Einstein condensate. This
provides an opportunity to distinguish axions from other forms of dark matter
on observational grounds. I show that if the dark matter is axions, tidal
torque theory predicts a specific structure for the phase space distribution of
the halos of isolated disk galaxies, such as the Milky Way. This phase space
structure is precisely that of the caustic ring model, for which observational
support had been found earlier. The other dark matter candidates predict a
different phase space structure for galactic halos.
