Severely Constraining Dark Matter Interpretations of the 21-cm Anomaly
  The EDGES Collaboration has recently reported the detection of a
stronger-than-expected absorption feature in the global 21-cm spectrum,
centered at a frequency corresponding to a redshift of z ~ 17. This observation
has been interpreted as evidence that the gas was cooled during this era as a
result of scattering with dark matter. In this study, we explore this
possibility, applying constraints from the cosmic microwave background, light
element abundances, Supernova 1987A, and a variety of laboratory experiments.
After taking these constraints into account, we find that the vast majority of
the parameter space capable of generating the observed 21-cm signal is ruled
out. The only range of models that remains viable is that in which a small
fraction, ~ 0.3-2%, of the dark matter consists of particles with a mass of ~
10-80 MeV and which couple to the photon through a small electric charge,
epsilon ~ 10^{-6}-10^{-4}. Furthermore, in order to avoid being overproduced in
the early universe, such models must be supplemented with an additional
depletion mechanism, such as annihilations through a L_{\mu}-L_{\tau} gauge
boson or annihilations to a pair of rapidly decaying hidden sector scalars.
