  The statistics of wide-separation (6'' < theta < 15'') gravitational lenses
constrain the amount of mass in the cores of dark matter halos on group and
cluster mass scales. For a family of halo models with a central cusp $\rho
\propto r^{-alpha}$ (1.0 <= alpha <= 1.9), the lack of wide-separation lenses
in the large Cosmic Lens All-Sky Survey yields an upper limit on the fraction
of the halo mass that is contained within ~4% of the virial radius, f_{core} <
0.023 (95% confidence level, LCDM). This limit offers an important test of the
cold dark matter paradigm. While the halo profiles derived from numerical
simulations appear to be consistent with this upper limit, larger surveys
currently underway such as the 2dF and SDSS should detect wide-separation
lenses and thus provide a measurement of the core mass fraction in massive dark
matter halos.
