  Similarly to warm dark matter which features a cut-off in the matter power
spectrum due to free-streaming, many interacting dark matter models predict a
suppression of the matter power spectrum on small length scales through
collisional damping. Forecasts for 21cm line intensity mapping have shown that
an instrument like the SKA will be able to probe a suppression of power in warm
dark matter scenarios in a statistically significant way. Here we investigate
the implications of these findings on interacting dark matter scenarios,
particularly dark matter-neutrino interactions, which we use as an example.
Using a suite of cosmological $N$-body simulations, we demonstrate that
interacting scenarios show a suppression of the non-linear power spectrum
similar to warm dark matter models. This implies that 21cm line intensity
mapping will be able to set the strongest limits yet on dark matter-neutrino
scattering, improving the constraints by two orders of magnitude over current
Lyman-$\alpha$ bounds, and by four orders of magnitude over cosmic microwave
background and baryon acoustic oscillations limits. However, to distinguish
between warm dark matter and interacting scenarios, our simulations show that
percent-level precision measurements of the matter power spectrum at redshifts
$z\gtrsim15$ are necessary, as the key features of interacting scenarios are
washed out by non-linear evolution at later times.
