  Gas-rich dwarf galaxies located outside the virial radius of their host are
relatively pristine systems and have ultra-low gas cooling rates. This makes
them very sensitive to heat injection by annihilation or decay of dark matter
(DM). Such dwarfs are particularly sensitive to DM producing e$^\pm$ with
energies 1$-$100 MeV or photons with energies 13.6 eV$-1$ keV, because these
products can be efficiently thermalized in the neutral hydrogen gas of the
dwarfs. Following the methodology of Wadekar and Farrar (2021), we require the
rate of heat injection by DM to not exceed the ultra-low radiative cooling rate
of gas in the Leo T dwarf galaxy. This gives model-independent bounds on $(i)$
the decay lifetime of DM to $e^\pm$ (photons) which are stronger than all the
previous literature for $m_\mathrm{DM}\sim$ 1$-$10 MeV
($m_\mathrm{DM}\sim0.02-1$ keV), $(ii)$ annihilation of DM to $e^{\pm}$
comparable to constraints from CMB and X/$\gamma$-ray surveys. We also
translate our bounds for the case of the following DM models: axion-like
particles (ALPs), sterile neutrinos, excited DM states, higgs portal scalars
and dark baryons. Observations of gas-rich low-mass dwarfs from upcoming 21cm
and optical surveys can therefore be powerful probes of a multitude of models
of DM.
