Constraints on the Collisional Nature of the Dark Matter from Gravitational Lensing in the Cluster A2218

The detailed distribution of mass within clusters of galaxies can be used to probe the nature of dark matter. We show that constraints on the extent of the mass distribution around galaxies in the rich cluster Abell 2218 obtained from combining strong and weak lensing observations are consistent with the predictions which assume that the dominant mass component (dark matter) in these halos is collisionless. A strongly interacting (fluid-like) dark matter is ruled-out at a confidence level of more than 5-sigma.