  The hypothesis of an `invisible' axion was made by Misha Shifman and others,
approximately thirty years ago. It has turned out to be an unusually fruitful
idea, crossing boundaries between particle physics, astrophysics and cosmology.
An axion with mass of order $10^{-5}$ eV (with large uncertainties) is one of
the leading candidates for the dark matter of the universe. It was found
recently that dark matter axions thermalize and form a Bose-Einstein condensate
(BEC). Because they form a BEC, axions differ from ordinary cold dark matter
(CDM) in the non-linear regime of structure formation and upon entering the
horizon. Axion BEC provides a mechanism for the production of net overall
rotation in dark matter halos, and for the alignment of cosmic microwave
anisotropy multipoles. Because there is evidence for these phenomena,
unexplained with ordinary CDM, an argument can be made that the dark matter is
axions.
