A fresh look into the interacting dark matter scenario
  The elastic scattering between dark matter particles and radiation represents
an attractive possibility to solve a number of discrepancies between
observations and standard cold dark matter predictions, as the induced
collisional damping would imply a suppression of small-scale structures. We
consider this scenario and confront it with measurements of the ionization
history of the Universe at several redshifts and with recent estimates of the
counts of Milky Way satellite galaxies. We derive a conservative upper bound on
the dark matter-photon elastic scattering cross section of $\sigma_{\gamma
\rm{DM}} < 8 \times 10^{-10} \, \sigma_T \, \left(m_{\rm DM}/{\rm GeV}\right)$
at $95\%$~CL, about one order of magnitude tighter than previous {constraints
from satellite number counts}. Due to the strong degeneracies with
astrophysical parameters, the bound on the dark matter-photon scattering cross
section derived here is driven by the estimate of the number of Milky Way
satellite galaxies. Finally, we also argue that future 21~cm probes could help
in disentangling among possible non-cold dark matter candidates, such as
interacting and warm dark matter scenarios. Let us emphasize that bounds of
similar magnitude to the ones obtained here could be also derived for models
with dark matter-neutrino interactions and would be as constraining as the
tightest limits on such scenarios.
