  We analyze a simple extension of the Standard Model (SM) with a dark sector
composed of a scalar and a fermion, both singlets under the SM gauge group but
charged under a dark sector symmetry group. Sterile neutrinos, which are
singlets under both groups, mediate the interactions between the dark sector
and the SM particles, and generate masses for the active neutrinos via the
seesaw mechanism. We explore the parameter space region where the observed Dark
Matter relic abundance is determined by the annihilation into sterile
neutrinos, both for fermion and scalar Dark Matter particles. The scalar Dark
Matter case provides an interesting alternative to the usual Higgs portal
scenario. We also study the constraints from direct Dark Matter searches and
the prospects for indirect detection via sterile neutrino decays to leptons,
which may be able to rule out Dark Matter masses below and around 100 GeV.
