$21 {cm}$ with millicharged dark matter.

We study scenarios where a sub-percent fraction of dark matter (DM) carries a millicharge (mDM). The small energy density of the millicharge component avoids the strong constraints from CMB but can have interesting effects on the cosmological evolution at and after recombination. For large enough charges and at small relative velocities, non-relativistic effects like Sommerfeld enhancement and bound state formation significantly impact the behavior of mDM. We systematically compute the scattering rates of mDM with hydrogen and helium and the rate of mDM capture in the interstellar medium. We discuss how these processes can leave an impact in the global $21 {cm}$ spectrum.