  We propose an extension of the standard model (SM) by including a dark sector
comprising of three generations of heavy right-handed neutrinos, a singlet
scalar and a singlet Dirac fermion, where the latter two particles are stable
and are viable candidates of dark matter (DM). In the early Universe, the
CP-violating out-of-equilibrium decay of heavy right-handed neutrinos to
singlet Dirac fermion and scalar in the dark sector generates a net DM
asymmetry. The latter is then transported to the visible sector via a dimension
eight operator which conserves $B-L$ symmetry and is in thermal equilibrium
above the sphaleron decoupling temperature. An additional light singlet scalar
is introduced which mixes with the SM Higgs and pave a path for annihilating
the symmetric components of the DM candidates. Then we discuss the constraints
on singlet-doublet Higgs mixing from invisible Higgs decay, signal strength at
LHC and direct search of DM at terrestrial laboratories. At tree level the
neutrinos are shown to be massless since the symmetry of dark sector forbids
the interaction of right-handed neutrinos with the SM particles. However, at
one loop level the neutrinos acquire sub-eV masses as required by the
oscillation experiments.
