  Gravitational lensing observations of massive X-ray clusters imply a steep
characteristic density profile marked by a central concentration of dark
matter. The observed mass fraction within a projected radius of 150 kpc is
twice that found in state-of-the-art dark matter simulations of the standard
Lambda-CDM cosmology. A central baryon enhancement that could explain this
discrepancy is not observed, leaving a major puzzle. We propose a solution
based on the merger histories of clusters. A significant fraction of the final
dark matter content of a cluster halo originates within galaxy-sized halos, in
which gas can cool and compress the dark matter core to high densities. The
subsequent tidal stripping of this compressed dark matter occurs in denser
regions that are closer to the center of the cluster halo. Eventually, the
originally cooled gas must be dispersed into the intracluster medium through
feedback, for consistency with observations that do not find central baryon
enhancements in clusters. Still, the early adiabatic compression of the
galactic dark matter leaves a net effect on the cluster. Using a simple model
for this process, we show that the central cluster profile is substantially
modified, potentially explaining the observed discrepancy.
