Abell 520 (also known as A520) is a galaxy cluster in the Orion constellation, located at a co-moving radial distance of 811 Mpc (2,645 Mly) and subtends 25 arcminutes on the Earth sky.

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 at first, 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.

While one study[9] 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[10] based on the Advanced Camera for Surveys (ACS) did not support the claim.

A subsequent analysis[13] by an independent third team of the gravitational shear catalogs of the two competing ACS analyses indicates marginal evidence for the core in both data sets and the authors "do not consider A520 as posing a significant challenge to the collisionless dark matter scenario."