Four-galaxy collision provides possible hint of dark matter interactions

BBC : The existence of dark matter has been established through observations of the effect its mass has on normal matter and on light via gravitational lensing. However, observations of dark matter have not provided any other clues about how it interacts with normal matter or itself other than through gravitation. Now, Richard Massey of Durham University in the UK and his colleagues say they may have found evidence of dark matter interacting with itself. They used gravitational lensing to examine a four-galaxy collision in the Abell 3827 cluster 1.4 billion light-years away. It appears that the dark matter surrounding one of the galaxies is lagging behind the rest of the matter in that galaxy. Massey’s group has studied other galactic collisions and not seen such a phenomenon. They admit that they have not ruled out all possible known astrophysical sources for the new discrepancy, and they intend to create models of the system to do so.