  Dark matter interactions with massless or very light Standard Model
particles, as photons or neutrinos, may lead to a suppression of the matter
power spectrum at small scales and of the number of low mass haloes. Bounds on
the dark matter scattering cross section with light degrees of freedom in such
interacting dark matter (IDM) scenarios have been obtained from e.g. early time
cosmic microwave background physics and large scale structure observations.
Here we scrutinize dark matter microphysics in light of the claimed 21 cm EDGES
78 MHz absorption signal. IDM is expected to delay the 21 cm absorption
features due to collisional damping effects. We identify the astrophysical
conditions under which the existing constraints on the dark matter scattering
cross section could be largely improved due to the IDM imprint on the 21 cm
signal, providing also an explicit comparison to the WDM scenario.
