  The addition of gauge singlet fermions to the Standard Model Lagrangian
renders the neutrinos massive and allows one to explain all that is
experimentally known about neutrino masses and lepton mixing. At the same time,
the gauge singlet fermion decays in the early universe produce a lepton
asymmetry, which is converted to a baryon asymmetry via Spharelon processes
(leptogenesis). On the other hand, the addition of a gauge singlet scalar to
the Standard Model yields a thermal dark matter candidate through interactions
between the Higgs boson and the gauge singlet scalar. By imposing a $Z_2$
symmetry on the gauge singlet scalar and one of the gauge singlet fermions, we
can have viable dark matter candidates and new interactions coupling the
$Z_2$-odd scalar to the $Z_2$-odd fermion, which can lower the leptogenesis
scale (and the reheating temperature) to $\mathcal{O}$(TeV).
