New Hubble observations puzzle astronomers.

An unusual patch of sky devoid of galaxies could hint at the existence of more than one type of dark matter, scientists say. Dark matter at the center of Abell 520, a cosmic "train wreck" of galaxy clusters located about 2.4 billion light-years away, is not behaving as predicted, according to new results from NASA's Hubble Space Telescope Dark matter is a mysterious substance that appears to make up almost a quarter of the universe but cannot be detected with current instruments. Theory predicts that dark matter clusters, or "halos," form gravitational anchors around which normal matter can coalesce to form galaxies. When galaxies and galaxy clusters crash into one another, their dark matter skeletons also collide. Because normal matter is gravitationally attracted to dark matter, the visible tatters of the colliding galaxies should be dragged along, like dogs on leashes. This is in fact what astronomers observed in 2006 in the Bullet Cluster (pictured), which is a textbook example of how dark matter should behave. During the same 2006 research, astronomers examining Abell 520 found that its dark matter is concentrated in a "dark core" that contains far fewer galaxies than would be expected if the dark matter and galaxies had stuck together. Most of the galaxies have apparently sailed far away from the collision. "It is very hard to explain this with current theories of dark matter or galaxy formation," said study co-author Myungkook James Jee at the University of California, Davis. Dark Matter Results Puzzling The 2006 results were so unexpected that some scientists dismissed them. But the recent Hubble analysis backs up those initial observations, leaving astronomers as puzzled as ever. "We tried to come up with models that would explain this, but there were not any good models," study co-author Andisheh Mahdavi, an astronomer at San Francisco State University, said in a statement. "There is no way that you could have cold dark matter piling up like this in a region with so few galaxies," said Mahdavi, who was involved in the 2006 observations. (See "Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?") The team has proposed several explanations for their findings, but none of them can be accounted for by current dark matter theory. One idea is that there are more than one type of dark matter, and that some varieties clump together during collisions while others do not. "If there is more than one species of dark matter, one could be collisionless and the other could be collisional," said study co-author Jee, whose study was recently published online in the Astrophysical Journal. "But that is all very speculative right now." Dark Matter Theories in Question? Another possibility is that Abell 520 involves a collision between three galaxy clusters, instead of just two systems, as in the case of the Bullet Cluster. The result would be a more complicated interaction, experts say. For instance, "one can imagine a transient structure exhibiting a dark matter peak without galaxies," said Avi Loeb, chair of the astronomy department at Harvard University. (See galaxy pictures.) Computer simulations of Abell 520 could help resolve the mystery by helping to calculate just how likely it is that Abell 520's dark matter core could exist, added Loeb, who was not involved in the study. "Without such a statistical analysis, it is premature to conclude that the observed structure is unlikely." The team says plans are already underway to conduct computer simulations to try to explain Abell 520. But if the simulations fail, particle physicists may have to revise their theories about dark matter, co-author Mahdavi said. "I'm just as perplexed as I was back in 2007," he added. "It's a pretty disturbing observation to have out there."