Was the Dark Matter Cold All Along?

We need to conjecture a cold or nonrelativistic form of dark matter in our niverse which dominates the matter energy density, to explain several cosmological observations. Although we do not yet know anything about its microscopic nature, dark matter may have been relativistic and collisional at early time, and became cold only after a phase transition as predicted in many particle physics models. In this talk, I shall describe and explain the nontrivial features of the cosmology of such a class of dark matters, dubbed Ballistic Dark Matter (BDM). The dark matter particles inherit large peculiar velocities from the primordial relativistic fluid, hence the name ballistic, that cause oscillations and enhancement of the matter power spectrum relative to the ΛCDM cosmology. I shall also discuss some qualitative constraints on BDM models from the large scale structure and CMB observation data.