The non-gravitational interactions of dark matter in colliding galaxy
  clusters
  Collisions between galaxy clusters provide a test of the non-gravitational
forces acting on dark matter. Dark matter's lack of deceleration in the `bullet
cluster collision' constrained its self-interaction cross-section \sigma_DM/m <
1.25cm2/g (68% confidence limit) for long-ranged forces. Using the Chandra and
Hubble Space Telescopes we have now observed 72 collisions, including both
`major' and `minor' mergers. Combining these measurements statistically, we
detect the existence of dark mass at 7.6\sigma significance. The position of
the dark mass has remained closely aligned within 5.8+/-8.2 kpc of associated
stars: implying a self-interaction cross-section \sigma_DM/m < 0.47 cm2/g (95%
CL) and disfavoring some proposed extensions to the standard model.
