The statistics of wide-separation ( 6 \arcsec < \theta < 15 \arcsec ) 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 \leq \alpha \leq 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 \sim 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 .