The Abell 520 galaxy cluster is an strange structure formed by a major merger. Due to its odd and chaotic nature it has been given the nick-name the Train Wreck Cluster.
The Dark Matter within the cluster does not act as expected like it does in other clusters, therefore, Abell 520 creates problems for many of the prevailing theories about Dark Matter. It also disrupts many alternative theories of modified gravity. Similar to the Bullet Cluster the gas contents and galaxies within the cluster are separated. The cluster is on par with others of its size for the amount of galaxies and intergalactic gases that it contains. However, within the large gravitational lensing core there seems to be a lack of galaxies and other normal matter. Most theories hold that Dark matter must stay closely attached to galaxies. The lack of galaxies where the Dark Matter should be is the reason that this cluster is a problem for many theories on Dark Matter. Most theories state that intergalactic gas can separate from both galaxies and dark matter during mergers. However, within Abell 520 there is no evidence that the core has any effect on the clusters other components. No reason has been stated why the dark matter and galaxies are separate.