"Within the DM paradigm the Tully-Fisher law must arise from galaxy formation since it connects luminosity of baryonic matter with a dynamical property, rotation, which is seen as dominated by the DM halo. But it has not been easy to derive Tully-Fisher from any natural connection between the two components. And as R. H. Sanders has pointed out, the messiness of galaxy formation is hardly the natural backdrop for such a sharp correlation between galaxy properties. The sharpness needs a dynamical reason as opposed to an evolutionary one."