Galactic Car Crash

Above: Abell 520   Green: Hot gas which is evidence of collision. Blue is where majority of mass is, which is dominated by dark matter. Starlight from galaxies, derived from observations by the Canada-France-Hawaii Telescope, is colored orange

It is not terribly uncommon for galaxies to collide with one another, but astronomers using data from the Hubble space telescope have observed a puddle of dark matter left behind from two massive cluster galaxies. `There is a very low density of stars within galaxies. The galaxies are filled with different gases and when two galaxies collide, it is likely that the stars pass by each other without interacting, but generally the gas collides and is left behind. the results found involving dark matter, however, could challenge current theories about dark matter that predict galaxies should be anchored to the invisible substance even during the shock of a collision. While dark matter may be invisible, the effects can be seen. Dark matter can bend light moving by, essentially magnifying objects behind it called lensing. This technique led to a puzzle when observing Abell 520 though; the dark matter in  had collected into a "dark core," containing far fewer galaxies than would be expected if the dark matter and galaxies were anchored together. This means that many of the galaxies originally with the dark matter has gone drifting somewhere else. This challenges our understanding of dark matter which means more research and innovative ideas will have to be created to better understand dark matter. The current problem however is that there are not very many collisions between local galaxies which is the primary way of studying how dark matter effects its surroundings. The other main galaxy cluster, the bullet cluster, is inconsistent with Abell 520. The dark matter and the galaxies bound to it were different in each case. The different instances could mean that "some" dark matter is sticky and smashes with dark matter- slowing it down, while other types of dark matter can pass through each other without interacting much. One other possible issue from abell 520 is the limitation of hubble. It is entirely possible that there are in fact galaxies within abell 520, but they are so relatively faint that hubble is not able to resolve them.