  Massive and extended dark halos can inhibit the formation of long tidal tails
in galaxy collisions. We examine this effect using an extensive survey of
simulations with different dark halo potentials to constrain halo properties of
interacting galaxies. These constraints are compared to other observational
limits and theoretical predictions of halo structure. The dark halos predicted
by $\Omega=1$ cosmological models like CDM are too massive and extended to
produce the long tidal tails seen in nearby galaxy collisions. There is also a
conflict with the halo potentials inferred from satellite kinematics; such
halos would likewise inhibit tail formation in galaxy collisions.
