http://antwrp.gsfc.nasa.gov/apod//ap070820.html "Conventional wisdom holds that dark matter and normal matter are attracted the same gravitationally, and so should be distributed the same in Abell 520. Inspection of the above image, however, shows a surprising a lack of a concentration of visible galaxies along the dark matter. One hypothetical answer is that the discrepancy is caused by the large galaxies undergoing some sort of conventional gravitational slingshots.
A more controversial hypothesis holds that the dark matter is colliding with itself in some non-gravitational way that has never been seen before. "
http://www.sciencenews.org/articles/20070825/fob5.asp "The Bullet cluster was a spectacular result, because it beautifully confirmed our assumptions about how dark matter, gas, and galaxies behave, [but] Abell 520 does the complete opposite," comments Julianne Dalcanton of the University of Washington in Seattle. One explanation for the new results is that dark matter is composed of particles that interact through forces other than gravity. However, such particles would cause a variety of other effects that have never been seen, such as making galaxy clusters spherical, notes Katherine Freese of the University of Michigan in Ann Arbor."
http://space.newscientist.com/article/dn12497-cosmic-train-wreck-defies-dark-matter-theories.html "Abell 520 turns out to hold a massive dark core, empty of bright galaxies. Some of the core is made up of hot gas, which the team detected from its emission of X-rays, but most of it has to be something else – presumably the same dark matter that astronomers detect elsewhere in the universe. ... snip ... One possibility is that the galaxies were once in the core, along with the dark matter, but then close encounters between the galaxies threw them out to the cluster's fringes. Unfortunately, the team can't get that to happen in their computer simulations, even if they tailor the initial conditions to encourage these gravitational slingshots. A more intriguing explanation is that when the original clusters collided, their dark matter was stripped out. Astronomers expect that to happen to gas clouds in colliding clusters, but dark matter is supposed to be more slippery, barely interacting with other matter or with itself. "We expect clouds of dark matter to flow right through each other," says team member Arif Babul at the University of Victoria in Canada. ... snip ... Could there be two types of dark matter, the conventional slippery form and another that interacts more strongly? Babul says it is possible, but dislikes the idea of invoking yet another invisible cosmic substance to explain these observations."
I'm sure they'll do it if they have to.
