  We consider the consequences of SIDM for a velocity dependent cross section
per unit mass. Accretion of SIDM onto seed black holes can produce supermassive
black holes that are too large for certain combinations of parameters,which is
used to obtain a new constraint on the dark matter interaction. Constraints due
to other considerations are presented and previous ones are generalized. The
black hole constraint is extremely sensitive to the slope \alpha, of the inner
density profile of dark halos. For the most probable value of \alpha=1.3, there
exists a narrow range in parameter space, such that all constraints are
satisfied. However, the adiabatic compression of the dark halo by baryons as
they cool and contract in normal galaxies yields a steeper cusp, \alpha=1.7.
This gives a tighter constraint, which would exclude SIDM as a possible
solution to the purported problems with CDM in the absence of other dynamical
processes. Nevertheless, SIDM with parameters consistent with this stronger
constraint, can explain the ubiquity of supermassive black holes in the centers
of galaxies. A ``best fit'' model is presented which reproduces the
supermassive black hole masses and their observed correlations with the
velocity dispersion of the host bulges. Specifically, the fourth power
dependence of black hole mass on velocity dispersion is a direct consequence of
the power spectrum having an index of n=-2. Although the dark matter collision
rates for this model are too small to directly remedy problems with CDM,
mergers between dark halos harboring supermassive black holes at high redshift
could ameliorate the cuspy halo problem. This scenario also explains the lack
of comparable supermassive black holes in bulgeless galaxies like M33.
