Repulsive Dark Matter
  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. 
Jeremy Goodman