  Accurate measurements of the mass distribution in galaxy and cluster halos
are essential to test the cold dark matter (CDM) paradigm. The cosmological
model predicts a universal shape for the density profile in all halos,
independent of halo mass. Its profile has a `cuspy' centre, with no evidence
for the constant density core. In this paper we carry out a careful analysis of
twelve galaxy clusters, using Chandra data to compute the mass distribution in
each system under the assumption of hydrostatic equilibrium. Due to their low
concentration, clusters provide ideal objects for studying the central cusps in
dark matter halos. The majority of the systems are consistent with the CDM
model, but 4 objects exhibit flat inner density profiles. We suggest that the
flat inner profile found for these clusters is due to an underestimation of the
mass in the cluster centre (rather than any problem with the CDM model), since
these objects also have a centrally peaked gas mass fraction. We discuss
possible causes for erroneously low mass measurements in the cores of some
systems.
