Dark Higgs Dark Matter
  A new $U(1)$ dark gauge group coupled to the Standard Model (SM) via the
kinetic mixing portal provides a natural dark matter candidate in the form of
the Higgs field, $h_d$, responsible for generating the mass of the dark photon,
$\gamma_d$. We show that the condition $m_{h_d}\leq m_{\gamma_d}$, together
with smallness of the kinetic mixing parameter, $\epsilon$, and/or dark gauge
coupling, $g_d$, leads the dark Higgs to be sufficiently metastable to
constitute dark matter. We analyze the Universe's thermal history and show that
both freeze-in, ${\rm SM}\to \{\gamma_d, h_d\}$, and freeze-out, $ \{\gamma_d,
h_d\} \to {\rm SM}$, processes can lead to viable dark Higgs dark matter with a
sub-GeV mass and a kinetic mixing parameter in the range
$10^{-13}\lesssim\epsilon\lesssim10^{-6}$. Observable signals in astrophysics
and cosmology include modifications to primordial elemental abundances, altered
energetics of supernovae explosions, dark Higgs decays in the late Universe,
and dark matter self-interactions.
