  We use observations of gas-rich dwarf galaxies to derive constraints on dark
matter scattering with ordinary matter. We require that heating/cooling due to
DM interacting with gas in the Leo T dwarf galaxy not exceed the ultra-low
radiative cooling rate of the gas. This enables us to set $(i)$ stronger bounds
than all the previous literature on ultra-light hidden photon DM for nearly all
of the mass range $10^{-23}\lesssim m_\mathrm{DM} \lesssim 10^{-10}$ eV, $(ii)$
limits on sub-GeV millicharged DM which add to the constraints on the recent
EDGES 21cm absorption anomaly, and $(iii)$ constraints on DM-baryon
interactions directly at low relative velocities $v_\mathrm{rel}\sim 17$ km/s.
Our study opens a new direction at using observations of gas-rich dwarf
galaxies from previous, current and upcoming optical and 21cm surveys to probe
physics beyond the standard model.
