  In the thermal dark matter (DM) paradigm, primordial interactions between DM
and Standard Model particles are responsible for the observed DM relic density.
In Boehm et al. (2014), we showed that weak-strength interactions between DM
and radiation (photons or neutrinos) can erase small-scale density
fluctuations, leading to a suppression of the matter power spectrum compared to
the collisionless cold DM (CDM) model. This results in fewer DM subhaloes
within Milky Way-like DM haloes, implying a reduction in the abundance of
satellite galaxies. Here we use very high resolution N-body simulations to
measure the dynamics of these subhaloes. We find that when interactions are
included, the largest subhaloes are less concentrated than their counterparts
in the collisionless CDM model and have rotation curves that match
observational data, providing a new solution to the "too big to fail" problem.
