  Recent, very accurate simulations of galaxy formation have revealed that the
standard cold dark matter model has great difficulty in explaining the detailed
structure of galaxies. One of the major problems is that galactic halos are too
centrally concentrated. Dark matter self-interactions have been proposed as a
possible means of resolving this inconsistency. Here, we investigate
quantitatively the effect of dark matter self interactions on formation of
galactic halos. Our numerical framework is extremely simple, while still
keeping the essential physics. We confirm that strongly self-interacting dark
matter leads to less centrally concentrated structures. Interestingly, we find
that for a range of different interaction strengths, the dark matter halos are
unstable to particle ejection on a timescale comparable to the Hubble time.
