1999MNRAS.308..551W

If dark haloes are composed of dense gas clouds, as has recently been inferred, then collisions between clouds lead to galaxy evolution. Collisions introduce a core in an initially singular dark matter distribution, and can thus help to reconcile scale-free initial conditions - such as are found in simulations - with observed haloes, which have cores. A pseudo-Tully-Fisher relation, between halo circular speed and visible mass (not luminosity), emerges naturally from the model: M_vis∝V^7/2.

Published data conform astonishingly well to this theoretical prediction. For our sample of galaxies, the mass-velocity relationship has much less scatter than the Tully-Fisher relation, and holds as well for dwarf galaxies (where diffuse gas makes a sizeable contribution to the total visible mass) as it does for giants. It seems very likely that this visible-mass/velocity relationship is the underlying physical basis for the Tully-Fisher relation, and this discovery in turn suggests that the dark matter is both baryonic and collisional.