On the structure and nature of dark matter halos

The structure of dark matter halos as predicted from cosmological models is discussed and compared with observed rotation curves of dark matter-dominated dwarf galaxies. The theoretical models predict that dark matter halos represent a one-parameter family with a universal density profile. Observations of dark matter-dominated rotation curves indeed confirm the universal structure of dark halos. They are even in excellent agreement with the expected mass-radius scaling relations for the currently favoured cosmological model (standard cold dark matter with Omega_0=0.25 and Omega_{Lambda}=0.75). The rotation profiles however disagree with the predicted dark matter density distributions. Secular processes which might affect the inner halo structure do not seem to provide a good solution to this problem. We discuss, as an alternative, the possibility that dark halos consist of two separate components, a dark baryonic and a dark non-baryonic component.