On dark atoms, massive dark photons and millicharged sub-components
  We present a simple model of two dark matter species with opposite
millicharge that can form electrically neutral bound states via the exchange of
a massive dark photon. If bound state formation is suppressed at low
temperatures, a sub-dominant fraction of millicharged particles remains at late
times, which can give rise to interesting features in the 21 cm absorption
profile at cosmic dawn. The dominant neutral component, on the other hand, can
have dipole interactions with ordinary matter, leading to non-standard signals
in direct detection experiments. We identify the parameter regions predicting a
percent-level ionisation fraction and study constraints from laboratory
searches for dark matter scattering and dark photon decays.
