  The epoch of the formation of the first stars, known as the cosmic dawn, has
emerged as a new arena in the search for dark matter. In particular, the first
claimed 21-cm detection exhibits a deeper global absorption feature than
expected, which could be caused by a low baryonic temperature, and has been
interpreted as a sign for electromagnetic interactions between baryons and dark
matter. This hypothesis has a striking prediction: large temperature
anisotropies sourced by the velocity-dependent cooling of the baryons. However,
in order to remain consistent with the rest of cosmological observations, only
part of the dark matter is allowed to be charged, and thus interactive. Here we
compute, for the first time, the 21-cm fluctuations caused by a charged
subcomponent of the dark matter, including both the pre- and post-recombination
evolution of all fluids. We find that, for the same parameters that can explain
the anomalous 21-cm absorption signal, any percent-level fraction of charged
dark matter would source novel 21-cm fluctuations with a unique acoustic
spectrum, and with an amplitude above any other known effects. These
fluctuations are uncorrelated with the usual adiabatic anisotropies, and would
be observable at high significance with interferometers such as LOFAR and HERA,
thus providing a novel probe of dark matter at cosmic dawn.
