  Despite the astonishing success of standard $\Lambda$CDM cosmology, there is
mounting evidence for a tension with observations at small and intermediate
scales. We introduce a simple model where both cold dark matter (DM) and
sterile neutrinos are charged under a new $U(1)_X$ gauge interaction. The
resulting DM self-interactions resolve the tension with the observed abundances
and internal density structures of dwarf galaxies. At the ame time, the sterile
neutrinos can account for both the small hot DM component favored by
cosmological observations and the neutrino anomalies found in short-baseline
experiments.
