The structural hierarchy of CDM implies that the Milky Way (MW) should have hundreds of dark matter satellites. However, only tens of satellite galaxies have been observed---a discrepancy termed the missing satellites problem. I showed that when accounting for invisible ``failed'' and yet unobserved faint galaxies, there are no missing satellites, and the structural hierarchy extends to at least to 10^8 solar masses.
This has crucial implications for dark matter model builders. New dark matter physics is not required to explain satellite abundances above this scale, ruling out dark matter candidates with a higher mass cutoff. Furthermore, neither completeness corrections nor reionization suppression of galaxy formation had been considered by many model builders. Including these effects can dramatically tighten constraints on or even rule out dark matter candidates. I outlined a methodology by which stronger constraints on dark matter models can be derived based on our new limit and demonstrated that the popular 7 keV sterile neutrino as well as warm dark matter thermal relics with a mass less than 4 keV are likely ruled out.
For more details, check out our paper There Is No Missing Satellites Problem.