The Nature Of Dark Matter
  Collisionless particles, such as cold dark matter, interact only by gravity and do not have any associated length scale, therefore the dark halos of galaxies should have negligible core radii. This expectation has been supported by numerical experiments of collisionless particles within scale free and cold dark matter cosmologies. Most dwarf spiral galaxies are almost completely dark matter dominated, allowing a unique insight into their mass - density profiles which can be approximated by isothermal spheres with core radii of order 3 - 7 kpc. We can therefore make a direct comparison between these galaxies, and halos which form within the numerical simulations. This yields a severe discrepancy in that the simulations predict the density to fall as rho(r) \propto r^{-1} on the scales where the data show that rho(r) = const: e.g. the models overestimate the mass within the central few kpc of the halos by a factor of four. The formation of the luminous component of galaxies exacerbates this disparity between theory and observations, since the contraction of the baryons can significantly increase the central dark matter density. 
Ben Moore