  The origin and composition of the cosmological dark matter remain a mystery.
However, upcoming 21-cm measurements during cosmic dawn, the period of the
first stellar formation, can provide new clues on the nature of dark matter.
During this era, the baryon-dark matter fluid is the slowest it will ever be,
making it ideal to search for dark matter elastically scattering with baryons
through massless mediators, such as the photon. Here we explore whether
dark-matter particles with an electric "minicharge" can significantly alter the
baryonic temperature and, thus, affect 21-cm observations. We find that the
entirety of the dark matter cannot be minicharged at a significant level, lest
it interferes with Galactic and extragalactic magnetic fields. However, if
minicharged particles comprise a subpercent fraction of the dark matter, and
have charges $\epsilon \sim 10^{-6}$---in units of the electron charge---and
masses $m_\chi \sim 1-60$ MeV, they can significantly cool down the baryonic
fluid, and be discovered in 21-cm experiments. We show how this scenario can
explain the recent result by the EDGES collaboration, which requires a lower
baryonic temperature than possible within the standard model, while remaining
consistent with all current observations.
