  We discuss the universal relation between density and size of observed Dark
Matter halos that was recently shown to hold on a wide range of scales, from
dwarf galaxies to galaxy clusters. Predictions of LambdaCDM N-body simulations
are consistent with this relation. We demonstrate that this property of
LambdaCDM can be understood analytically in the secondary infall model.
Qualitative understanding given by this model provides a new way to predict
which deviations from LambdaCDM or large-scale modifications of gravity can
affect universal behavior and, therefore, to constrain them observationally.
