Galaxy halos of cold dark matter acquire a universal density profile. 29 This yields a flat rotation curve over a substantial range of radius, and gives an excellent fit to observational data on massive galaxy rotation curves. There is a central density cusp ( 1 / r) which in normal galaxies is embedded in a baryonic disk, the inner galaxy being baryon-dominated.
Low surface brightness dwarf spiral galaxies provide a laboratory where one can study dark matter at all radii: even the central regions are dark matter-dominated. One finds that there is a soft, uniform density dark matter core in these dwarf galaxies. 25 It is still controversial whether the CDM theory can reproduce soft cores in dwarf galaxies: at least one group finds in high resolution simulations that the core profiles are even steeper than r-1, and have not converged.28
Disk sizes provide an even more stringent constraint on theoretical models. Indeed disk scale lengths cannot be explained. 29 ,30 The difficulty lies in the fact that if angular momentum is conserved as the baryons contract within the dark halos, approximately the appropriate amount of angular momentum is acquired by tidal torques between neighbouring density flutuations to yield correct disk sizes However simulations fail to confirm this picture. In practice, cold dark matter and the associated baryons are so clumpy that massive clumps fall into the center via dynamical friction and angular momentum is transferrd outwards. Disk torquing by dark matter clumps also plays a role. The result is that the final baryonic disks are far too small. The resolution presumably lies in gas heating associated with injection of energy into the gas via supernovae once the first massive stars have formed. 31 ,32