  We compare the central mass concentration of Cold Dark Matter halos found in
cosmological N-body simulations with constraints derived from the Milky Way
disk dynamics and from the Tully-Fisher relation. For currently favored values
of the cosmological parameters ($\Omega_0 \sim 0.3$; $\Lambda_0=1-\Omega_0 \sim
0.7$; $h \sim 0.7$; COBE- and cluster abundance-normalized $\sigma_8$; Big-Bang
nucleosynthesis $\Omega_b$), we find that halos with circular velocities
comparable to the rotation speed of the Galaxy have typically {\it three times}
more dark matter inside the solar circle than inferred from observations of
Galactic dynamics. Such high central concentrations of dark matter on the scale
of galaxy disks also imply that stellar mass-to-light ratios much lower than
expected from population synthesis models must be assumed in order to reproduce
the zero-point of the Tully-Fisher relation. Indeed, even under the extreme
assumption that {\it all} baryons in a dark halo are turned into stars, disks
with conventional $I$-band stellar mass-to-light ratios ($M/L_I \sim 2 \pm 1
(M/L_I)_{\odot}$) are about two magnitudes fainter than observed at a given
rotation speed. We examine several modifications to the $\Lambda$CDM model that
may account for these discrepancies and conclude that agreement can only be
accomplished at the expense of renouncing other major successes of the model.
Reproducing the observed properties of disk galaxies thus appears to demand
substantial revision to the currently most successful model of structure
formation.
