  We study the evolution of Hernquist profile ``galaxies'' in the presence of
self-interacting dark matter (SIDM), where the properties of the dark matter
can be parameterized by one number, sigma-hat = sigma M/a^2 for a halo of mass
M and break radius a. While the halos form constant density cores of size a/2
on the core radius relaxation time scale core collapse begins shortly
thereafter and a steeper 1/r^2 central density cusp starts forming faster than
predicted by 2-body relaxation. The formation of the steeper central cusp is
accelerated if the cooling baryons adiabatically compress the dark matter. The
natural consequence of SIDM is to exacerbate rather than to mitigate
astrophysical problems created by dark matter density cusps.
