  High resolution N-body simulations show that the density profiles of dark
matter halos formed in the standard CDM cosmogony can be fit accurately by
scaling a simple ``universal'' profile. Regardless of their mass, halos are
nearly isothermal over a large range in radius, but significantly shallower
than $r^{-2}$ near the center and steeper than $r^{-2}$ in the outer regions.
The characteristic overdensity of a halo correlates strongly with halo mass in
a manner consistent with the mass dependence of the epoch of halo formation.
Matching the shape of the rotation curves of disk galaxies with this halo
structure requires (i) disk mass-to-light ratios to increase systematically
with luminosity, (ii) halo circular velocities to be systematically lower than
the disk rotation speed, and (iii) that the masses of halos surrounding bright
galaxies depend only weakly on galaxy luminosity. This offers an attractive
explanation for the puzzling lack of correlation between luminosity and
dynamics in observed samples of binary galaxies and of satellite companions of
bright spiral galaxies, suggesting that the structure of dark matter halos
surrounding bright spirals is similar to that of cold dark matter halos.
