Most physicists think dark matter exists because large structures in the universe appear to be held together by the gravitational attraction of much more mass than we can see through telescopes. One way to test theories of dark matter is to study cluster mergers, which are collisions between galaxy clusters after they have steadily gravitated towards each other. Cluster mergers are also a testing ground for alternative theories of gravitation, such as modified Newtonian dynamics (MOND), that eschew the possibility of dark matter altogether.

Observations of the Abell 520 cluster by Andisheh Mahdavi and colleagues at the University of Victoria, together with Peter Capak from the California Institute of Technology, however, seem to be inexplicable using either dark-matter or alternative-gravity theories.

The researchers used data taken from the Canada-France-Hawaii telescope and the Subaru telescope in Hawaii, along with data from the Chandra X-ray telescope, to see how gravity in the Abell 520 cluster acted as a lens to bend light passing through it on the light’s journey to Earth. Using this “gravitational lensing” technique, they mapped the distribution of the three components of the cluster: galaxies, prevalent hot gas and dark matter.

Mahdavi and colleagues discovered a core of dark matter and hot gas, with a bound group of galaxies separated to one side. This goes against accepted “collisionless” dark-matter theories because both the galaxies and the dark matter should have remained unimpeded in the collision - in other words, they should be in the same place. Although the observations could be explained by using a “collisional” dark matter theory, this would not simultaneously be able to explain other cluster mergers, such as the Bullet Cluster, that are already described well by the collisionless theories.

The researchers also say that they could not account for the observations using MOND. However, Hong-Sheng Zhao - a physicist from St Andrews University in Scotland who was part of a group that explained the dynamics of the Bullet Cluster using a relativistic alternative-gravity theory called TeVeS - told that this might be because current simulations of MOND tend to ignore a subtle time-dependent effect of the gravity field. By including this effect in future simulations, he says, both the Bullet Cluster and the Abell 520 cluster could have the chance to be explained with an alternative-gravity theory. “Right now it is very curious,” he said.