As part of James Bullock's dissertation research, we found that the distribution of specific angular momentum in dark matter halos has a universal profile . But if the baryons have the same angular momentum distribution as the dark matter, this implies that there is too much baryonic material with low angular momentum to form the observed rotationally supported exponential disks [101, 102]. It has long been assumed (e.g. [43, 44]) that the baryons and dark matter in a halo start with a similar distribution, based on the idea that angular momentum arising from large-scale tidal torques will be similar across the entire halo. But as my colleagues and I argued recently, a key implication of our new picture of angular momentum growth by merging  is that the DM and baryons will get different angular momentum distributions. For example, the lower density gas will be stripped by pressure and tidal forces from infalling satellites, and in big mergers the gaseous disks will partly become tidal tails. Feedback is also likely to play an important role, and Maller and Dekel (in preparation) have shown using a simple model that this can account for data on the angular momentum distribution in low surface brightness galaxies .
A related concern is that high-resolution hydrodynamical simulations of galaxy formation lead to disks that are much too small, evidently because formation of baryonic substructure leads to too much transfer of angular momentum and energy from the baryons to the dark matter . But if gas cooling is inhibited in the early universe, more realistic disks form , more so in CDM than in m = 1 CDM . Hydrodynamical simulations also indicate that this disk angular momentum problem may be resolved if small scale power is suppressed because the dark matter is warm rather than cold , which I discuss next.