  High-resolution N--body simulations show a remarkable similarity in the
structure of dark matter halos formed through dissipationless hierarchical
clustering. Independent of halo mass, power spectrum, and cosmological
parameters, the density profiles of dark halos can be accurately fit by scaling
a simple two-parameter profile. The two parameters of the fit, which can be
expressed as the mass and characteristic density of each halo, are strongly
correlated. The characteristic density is directly proportional to the mean
density of the universe at the time of assembly of each halo. Low mass halos
have higher characteristic densities, reflecting their higher collapse
redshift. Using this halo structure to fit the rotation curves of a large
sample of disk galaxies, we show that halos formed in the standard cold dark
matter (CDM) scenario are too centrally concentrated to be consistent with the
rotation curves of low-surface brightness galaxies. Less concentrated halos,
such as those formed in a low-density, flat CDM universe ($\Omega_0=0.3$,
$\Lambda=0.7$), are favored by these observations. Our fits also indicate that
the dark halo is important at all radii, and that its total mass is strongly
correlated with the luminosity of the galaxy. This model provides a natural
explanation for the disk-halo conspiracy and for the lack of dependence of the
Tully-Fisher relation on galaxy surface brightness.
