  In these lectures I describe a theory of dark matter superfluidity developed
in the last few years. The dark matter particles are axion-like, with masses of
order eV. They Bose-Einstein condense into a superfluid phase in the central
regions of galaxy halos. The superfluid phonon excitations in turn couple to
baryons and mediate a long-range force (beyond Newtonian gravity). For a
suitable choice of the superfluid equation of state, this force reproduces the
various galactic scaling relations embodied in Milgrom's law. Thus the dark
matter and modified gravity phenomena represent different phases of a single
underlying substance, unified through the rich and well-studied physics of
superfluidity.
