March 2, 2012: Astronomers observed what appeared to be a clump of dark matter left behind during a bizarre wreck between massive clusters of galaxies. The dark matter collected into a "dark core" containing far fewer galaxies than would be expected if the dark matter and galaxies hung together. Most of the galaxies apparently have sailed far away from the collision. This result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to the invisible substance, even during the shock of a collision.
The initial observations, made in 2007, were so unusual that astronomers shrugged them off as unreal, due to poor data. However, new results obtained in 2008 from NASA's Hubble Space Telescope confirm that dark matter and galaxies parted ways in the gigantic merging galaxy cluster called Abell 520, located 2.4 billion light-years away. Now, astronomers are left with the challenge of trying to explain dark matter's seemingly oddball behavior in this cluster.See the rest: