  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.
