  We study how the internal structure of dark halos is affected if Cold Dark
Matter particles are assumed to have a large cross-section for elastic
collisions. We identify a cluster halo in a large cosmological N-body
simulation and resimulate its formation with progressively increasing
resolution. We compare the structure found in the two cases where dark matter
is treated as collisionless or as a fluid. For the collisionless case our
results agree with those of other workers. Collisional dark matter results in a
cluster which is more nearly spherical and has a more singular central density
profile. Substructure within the cluster is only weakly suppressed relative to
the collisionless case. The observed structure of dwarf galaxies argues against
self-interacting dark matter if, as seems likely, intermediate cross-sections
produce structure lying between the extremes we have simulated.
