  We study dark matter(DM) in the model with one keV scale right-handed
neutrino $\nu_{R1}$ and two GeV scale right-handed neutrinos $\nu_{R2,3}$, the
$\nu$SM. We find that one of the GeV scale right-handed neutrinos can have much
longer lifetime than the other when two GeV scale right-handed neutrinos are
degenerate. We show that mass and mixing of light neutrinos can be explained in
this case. Significant entropy release can be generated in a reheating produced
by the decay of one of the GeV scale $\nu_{R}$. The density of $\nu_{R1}$ DM
can be diluted by two orders of magnitude and the mixing of $\nu_{R1}$ with
active neutrinos is allowed to be much larger, reaching the bound from X-ray
observation. This mixing can lead to sizeable rate of $\nu_{R1}$ capture by
radioactive nuclei. The $\nu_{R1}$ capture events are mono-energetic electrons
with keV scale energy away from the beta decay spectrum. This is a new way to
detect DM in the universe.
