Reviving Millicharged Dark Matter for 21-cm Cosmology
  The existence of millicharged dark matter (mDM) can leave a measurable
imprint on 21-cm cosmology through mDM-baryon scattering. However, the minimal
scenario is severely constrained by existing cosmological bounds on both the
fraction of dark matter that can be millicharged and the mass of mDM particles.
We point out that introducing a long-range force between a millicharged
subcomponent of dark matter and the dominant cold dark matter (CDM) component
leads to efficient cooling of baryons in the early universe, while also
significantly extending the range of viable mDM masses. Such a scenario can
explain the anomalous absorption signal in the sky-averaged 21-cm spectrum
observed by EDGES, and leads to a number of testable predictions for the
properties of the dark sector. The mDM mass can then lie between 10 MeV and a
few hundreds of GeVs, and its scattering cross section with baryons lies within
an unconstrained window of parameter space above direct detection limits and
below current bounds from colliders. In this allowed region, mDM can make up as
little as $10^{-8}$ of the total dark matter energy density. The CDM mass
ranges from 10 MeV to a few GeVs, and has an interaction cross section with the
Standard Model that is induced by a loop of mDM particles. This cross section
is generically within reach of near-future low-threshold direct detection
experiments.
