  We consider the cosmology and phenomenology of a dark photon portal to a
simple dark sector consisting of a single, light, fermionic dark matter
particle species with mass in the MeV range. We entertain three possible
kinetic mixing structures of a new Abelian gauge group U(1)$_{\text{dark}}$
with the visible sector through U(1)$_{\text{e.m.}}$, U(1)$_{\text{Y}}$ and
T[SU(2)$_{\text{L}}$]. We assume the dark photon to be massive and around the
MeV scale, thus close to the mass scale of the dark matter candidate. We
compute the dark matter relic density via freeze-out and freeze-in,
entertaining the additional possibilities of (i) a late inflationary period
that could dilute the dark matter yield of heavy candidates, and (ii)
additional production modes, for models with under-abundant thermal production.
We explore the parameter space compatible with a variety of experimental and
astrophysical bounds, and discuss prospects for discovery with new CMB probes
and MeV gamma-ray telescopes.
