High-resolution N-body simulations of hierarchical clustering in a wid
e variety of cosmogonies show that the density profiles of dark matter
halos are universal, with low-mass halos being denser than their more
massive counterparts. This mass-density correlation is interpreted as
reflecting the earlier typical formation time of less massive objects
. We investigate this hypothesis in the light of formation times defin
ed as the epoch at which halos experience their last major merger. Suc
h halo formation times are calculated by means of a modification of th
e extended Press & Schechter formalism that includes a phenomenologica
l frontier, Delta(m), between tiny and notable relative mass captures
leading to the distinction between merger and accretion. For Delta(m)
similar to 0.6, we confirm that the characteristic density of halos is
essentially proportional to the mean density of the universe at their
time of formation. Yet, proportionality with respect to the critical
density yields slightly better results for open universes. In addition
, we find that the scale radius of halos is also essentially proportio
nal to their virial radius at the time of formation. We show that thes
e two relations are consistent with the following simple scenario. Vio
lent relaxation caused by mergers rearranges the structure of halos le
ading to the same density profile with universal values of the dimensi
onless characteristic density and scale radius. Between mergers, halos
grow gradually through the accretion of surrounding layers by keeping
their central parts steady and expanding their virial radius as the c
ritical density of the universe diminishes.