  High resolution cosmological N-body simulations show that the density
profiles of dark matter halos in hierarchical cosmogonies are universal, with
low mass halos typically denser than more massive ones. This mass-density
correlation is interpreted as reflecting the earlier formation of less massive
objects. We investigate this hypothesis in the light of formation times defined
as the epoch at which halos experience their last major merger. We find that
the characteristic density and the scale radius of halos are essentially
proportional, respectively, to the critical density of the universe and the
virial radius at the time of their formation. These two relations are
consistent with the following simple evolutionary picture. Violent relaxation
caused by major mergers rearrange the structure of halos leading to a universal
dimensionless density profile. Between major mergers, halos gradually grow
through the accretion of surrounding layers by keeping the central part steady
and only expanding their virial radius as the critical density of the universe
diminishes.
