  If dark halos 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 halos, which have cores. A pseudo-Tully-Fisher
relation, between halo circular speed and visible mass (not luminosity),
emerges naturally from the model: visible mass proportional to V^3.5.
  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.
