Most dark matter candidates cannot be distinguished by observations on large scales. Although the observed universe appears consistent with hierarchical structure formation, this still leaves a wide range of potential dark matter candidates. For example, the clustering properties of galaxies, abundances of rich clusters or even halo masses and sizes are all very similar in universes with matter density dominated by cold dark matter, warm dark matter or collisional dark matter. We therefore seek tests of the nature of the dark matter that are sensitive to its interaction properties and small scale power which manifests itself on non-linear scales.
Cold dark matter (CDM) halos form via a complicated sequence of hierarchical mergers that lead to a global structure set primarily by violent relaxation. Numerical simulations have played an important role in determining the shape and scaling of CDM halo profiles that have subsequently lead to new observational tests of the model. A single functional form can fit CDM halos from a mass scale of 107 M - 1015 M, where the density at a fixed fraction of the virial radius is higher in lower mass halos and the central profiles have steep singular cusps (c.f. Dubinski & Carlberg 1991, Warren etal 1992, Navarro etal 1996, Fukushige & Makino 1997, Moore etal 1998, Jing 2000 etc).
Galaxy clusters form via a similar process as individual galaxies, however most of the galactic fragments that formed clusters have survived the hierarchical growth, whereas on galaxy scales we find little trace of the merging hierarchy. Only a dozen satellites orbit the Milky Way, whereas a thousand satellites orbit within the Coma cluster. Numerical simulations of CDM halo formation have revealed that the abundance of dark matter subhalos within a galaxy is the same as found within a scaled galaxy cluster (Moore etal 1999, Klypin etal 1999). There are two solutions to this problem: (i) CDM is incorrect and the nature of the dark matter suppresses the formation of substructure halos, (ii) CDM is correct and the dark matter satellites of the Milky Way are present but only a few percent of them formed stars. Here I focus on the latter possibility.