  One contribution to any dark sector's abundance comes from its gravitational
production during inflation. If the dark sector is weakly coupled to the
inflaton and the Standard Model, this can be its only production mechanism. For
non-interacting dark sectors, such as a free massive fermion or a free massive
vector field, this mechanism has been studied extensively. In this paper we
show, via the example of dark massive QED, that the presence of interactions
can result in a vastly different mass for the dark matter (DM) particle, which
may well coincide with the range probed by upcoming experiments.
  In the context of dark QED we study the evolution of the energy density in
the dark sector after inflation. Inflation produces a cold vector condensate
consisting of an enormous number of bosons, which via interesting processes -
Schwinger pair production, strong field electromagnetic cascades, and plasma
dynamics - transfers its energy to a small number of "dark electrons" and
triggers thermalization of the dark sector. The resulting dark electron DM mass
range is from 50 MeV to 30 TeV, far different from both the $10^{-5}$ eV mass
of the massive photon dark matter in the absence of dark electrons, and from
the $10^9$ GeV dark electron mass in the absence of dark photons. This can
significantly impact the search strategies for dark QED and, more generally,
theories with a self-interacting DM sector. In the presence of kinetic mixing,
a dark electron in this mass range can be searched for with upcoming direct
detection experiments, such as SENSEI-100g and OSCURA.
