DAVIS, Calif., March 2 (UPI) -- U.S. astronomers say an observation challenges current theories predicting galaxies should be anchored to dark matter even when they collide with one another.
Data from NASA's Hubble Telescope shows a clump of dark matter left behind from a wreck between massive clusters of galaxies known as Abell 520 has collected into a "dark core" containing far fewer galaxies than would be expected if the dark matter and galaxies were anchored together.
Most of the galaxies apparently have sailed far away from the collision, leaving their dark matter behind, a NASA release said Friday.
"This result is a puzzle," astronomer James Jee of the University of California, Davis, said. "Dark matter is not behaving as predicted, and it's not obviously clear what is going on. It is difficult to explain this Hubble observation with the current theories of galaxy formation and dark matter."
When galaxy clusters crash, astronomers said, they expect to see galaxies tagging along with the dark matter, like a dog on a leash.
Abell 520 shows dark matter's behavior may not be so simple, they said.
The system's core is rich in dark matter but contained no luminous galaxies that normally would be seen in the same location as the dark matter.
"We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped," Jee said. "But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other. No single theory explains the different behavior of dark matter in those two collisions. We need more examples."