  We propose a ultraviolet complete theory for cold dark matter(CDM) and
sterile neutrino that can accommodate both cosmological data and neutrino
oscillation experiments at $1\sigma$ level. A new $U(1)_X$ dark gauge symmetry
is introduced, and is assumed to be broken at $\sim \mathcal{O}$(MeV) scale.
Such a light mediator for DM's self-scattering and scattering-off sterile
neutrinos can resolve three controversies for cold DM on small cosmological
scales, cusp vs. core, too-big-to-fail and missing satellites problems. We can
also accommodate $\sim$ eV scale sterile neutrinos as the hot dark matter(HDM)
and can fit some neutrino anomalies from neutrino oscillation experiments
within $1\sigma$. Finally the right amount of HDM can make a sizable
contribution to dark radiation, and also helps to reconcile the tension between
the data on the tensor-to-scalar ratio reported by Planck and BICEP2
Collaborations.
