  Dark-matter halos are the scaffolding around which galaxies and clusters are
built. They form when the gravitational instability of primordial density
fluctuations causes regions which are denser than average to slow their cosmic
expansion, recollapse, and virialize. Objects as different in size and mass as
dwarf spheroidal galaxies and galaxy clusters are predicted by the CDM model to
have halos with a universal, self-similar equilibrium structure whose
parameters are determined by the halo's total mass and collapse redshift. These
latter two are statistically correlated, however, since halos of the same mass
form on average at the same epoch, with small-mass objects forming first and
then merging hierarchically.
  The structural properties of dark-matter dominated halos of different masses,
therefore, should reflect this statistical correlation, an imprint of the
statistical properties of the primordial density fluctuations which formed
them. Current data reveal these correlations, providing a fundamental test of
the CDM model which probes the shape of the power spectrum of primordial
density fluctuations and the cosmological background parameters.
