Numerical studies of the formation of cold dark matter haloes have produced several robust results that allow unique tests of the hierarchical clustering paradigm. Universal properties of haloes, including their mass profiles and substructure properties, are being tested against observational data from the scales of dwarf galaxies to galaxy clusters. Resolving the fine-grained structure of haloes has enabled us to make predictions for ongoing and planned direct and indirect dark matter detection experiments taking us beyond the smooth spherical isotropic model for the Galactic halo.

From cold collapse to hierarchical clustering – a brief history

N-body simulations of the gravitational collapse of a collisionless system of particles pre-date the CDM model. Early simulations in the 1960s studied the formation of elliptical galaxies from the collapse of a cold top-hat perturbation of stars [1089; 1556; 1889]. The resulting virialization process gave rise to equilibrium structures with de Vaucouleurs [633] or Einasto [741; 743] type density profiles. Profiles of the same form but with higher concentrations are widely used to describe the light distribution of elliptical galaxies. It is remarkable that the end state of almost any gravitational collapse, independent of the small-scale structure and hierarchical merging pattern, leads to a similar global structure of the final equilibrium system [1143; 1426; 1474].

Computer simulations in the 1970s attempted to follow the expansion and collapse of a spherical overdensity to relate to the observed properties of virialized structures such as galaxy clusters [1930].