Galaxy clusters, like the universe as a whole, are composed of both baryonic (“ordinary”) matter and dark matter, with about 1 part in 6 of the former, and five parts in 6 of the latter. A curious thing happens – usually – when two clusters collide. Both the visible baryonic matter, consisting mainly of luminous galaxies, and the dark matter, continue on their way after the collision. They show almost no signs of the encounter, although they may be fated, ultimately, to merge.
However, there appears to be substantial interaction of the intergalactic gas that existed before the encounter in both clusters. As a result, the gas loses most of its momentum and is left behind, between the two departing clusters, after the collision. This gas is very hot, and is easily detected at X-ray wavelengths. The best known example of this phenomenon is the Bullet Cluster, about which research was reported in 2006. (News articles: here, here, here, here; Research: here).
Yet now there is confirmation of a galaxy cluster collision – Abell 520 (A520) – in which a large quantity of dark matter appears to have been left behind. This is surprising, since the main reason given to explain why this should not happen is that dark matter is expected to interact very little with itself. The magnitude of this interaction is expressed as the ratio of the interaction “cross section” to mass. In the Bullet Cluster, the upper limit for this was found to be at most 1 cm2g-1. But in A520 this same measure is estimated at ~3.8 cm2g-1.
A520 seems to be the exceptional case, rather than the the Bullet Cluster, since results similar to the latter have been found in other cluster collisions, such as MACS J0025.4-1222 (News article: here; Research: here), Abell 2744 (News article: here; Research: here), and DLSCL J0916.2+2951.