The Abell 520 galaxy cluster possesses an unusual substructure resulting from a major merger. It has been popularly nicknamed The Train Wreck Cluster, due to its chaotic structure, and is classified as a Bautz-Morgan type III cluster. It is at a co-moving radial distance of 811 Mpc (2,645 Mly) and subtends 25 arcminutes on the sky. Analysis of the motions of 293 galaxies in the cluster field suggested that Abell 520 was a cluster forming at the crossing of three filaments of the large scale structure
The surprising substructure of Abell 520 was reported in 2007 from a weak gravitational lensing study based on Canada-France-Hawaii-Telescope (CFHT) imaging data. It was surprising because the study found the "dark core" with a significant amount of mass in the region, where there is no concentration of bright cluster galaxies. No conventional understanding of dark matter can explain this peculiar concentration of dark matter. One interesting interpretation is that the substructure may arise from non-gravitational interaction of dark matter.
However, in the year 2012 two international teams of astronomers published conflicting results on Abell 520. While one study based on the Wide Field Planetary Camera 2 (WFPC2) on Hubble Space Telescope (HST) confirmed the previous claim of the dark core in Abell 520, the other study based on the Advanced Camera for Surveys (ACS) did not support the claim.
In 2014, the conflicts were resolved by the study, which analyzed the data from both teams and showed that in fact the ACS data also confirm the presence of the dark core. As of 2016, the "dark core" mystery in Abell 520 still remains.