Revealing the nature of dark matter is fundamental to cosmology and particle physics. A combination of observations and theory suggests that the dark matter consists of non-baryonic particles, and in this large class of hierarchical cosmogonic models a universe dominated by cold dark matter (CDM) remains plausible albeit with some potential problems on small scales.
The only convincing method for confirming the existence of non-baryonic dark matter is by direct detection. Many such experiments are in progress and are beginning to probe the parameter space allowed by cosmological and particle physics constraints (e.g. DAMA, CDMS). Direct and indirect detection experiments are highly sensitive to the local density of particles and their velocity distribution. For example, the flux of gamma-rays on Earth from neutralino annihilations in the galactic halo depends on the amount of substructure in the dark matter (e.g. Bergstrom etal 1999). It is therefore crucial to understand the phase space structure of galactic halos in hierarchical models in order that experiments can be fine tuned to search for the appropriate signals, and that event rates or modulation effects can be correctly interpreted (Ullio etal 2000).
Many of the ongoing dark matter searches adopt the principle that CDM particles passing through the solar system have a smooth continuous density distribution with isotropic Maxwellian velocities. Other halos models have been studied e.g. Sikivie (1992, 1999), who assume axially symmetric and cold collapse of matter to infer the presence of caustic rings in the solar neighbourhood. We can now use the results of high resolution computer simulations of structure formation within a CDM universe to examine these assumptions more carefully.