Is a simple collisionless relic dark matter particle ruled out?

The central densities of dark matter (DM) halos are much lower than predicted in cold DM models of structure formation. Confirmation that they have cores with a finite central density would allow us to rule out many popular types of collisionless particles as candidates for DM. Any model that leads to cusped halos (such as cold DM) is already facing serious difficulties on small scales, and hot DM models have been excluded. Here I show that fermionic warm DM is inconsistent with the wide range of phase-space densities in the DM halos of well-observed nearby galaxies.