  We present new spectroscopic observations of the gravitational arcs and the
brightest cluster galaxy (BCG) in the cluster MS2137-23 (z=0.313) obtained with
the Echelle Spectrograph and Imager on the Keck II telescope. We find that the
tangential and radial arcs arise from sources at almost identical redshifts
(z=1.501,1.502). We combine the measured stellar velocity dispersion profile of
the BCG with a lensing analysis to constrain the distribution of dark and
stellar matter in the central 100 kpc of the cluster. Our data indicate a
remarkably flat inner slope for the dark matter profile, rho_d propto r^-beta,
with beta<0.9 at 99% CL. Steep inner slopes obtained in cold dark matter
cosmological simulations -- such as Navarro Frenk & White (beta=1) or Moore
(1.5) universal dark matter profiles -- are ruled out at better than 99% CL. As
baryon collapse is likely to have steepened the dark matter profile from its
original form, our data provides a powerful test of the cold dark matter
paradigm at the cluster mass scale.
