  It seems necessary to suppress, at least partially, the formation of
structure on subgalactic scales. As an alternative to warm or collisional dark
matter, I postulate a condensate of massive bosons interacting via a repulsive
interparticle potential, plus gravity. This leads to a minimum lengthscale for
bound objects, and to superfluidity. Galactic dynamics may differ significantly
from that of more generic dark matter in not unwelcome ways, especially in the
core. Such particles can be realized as quanta of a relativistic massive scalar
field with a quartic self-interaction. At high densities, the equation of state
has the same form as that of an ideal relativistic gas despite the
interactions. If the nonrelativistic lengthscale is of order a kiloparsec, then
the energy density in these particles was comparable to that of photons at
early times, but small enough to avoid conflict with primordial
nucleosynthesis.
