The Hubble Space Telescope allowed scientists to observe what looks like a clump of dark matter which is what remains after a cosmic catastrophe that led to a merger of galaxy clusters. The result is called the Abell 520 cluster, an abnormal cosmic structure which has a chaotic nature exactly because it’s the result of a collision between galaxies.

In the Abell 520 cluster, which is about 2.4 billion light years away, dark matter wasn’t directly observed because it’s invisible but through its effects. The search for dark matter in fact is done mainly observing the distortion of light from galaxies behind it, an effect called gravitational lensing.

Observing those gravitational effects in the Abell 520 cluster it turned out that dark matter formed a dark core which seems to be separated from most of the galaxies that make up the cluster. The problem is that so far scientists thought that dark matter was closely connected to galaxies.

The Abell 520 cluster was discovered in 2007 and from the beginning the anomalous distribution of dark matter from the galaxies that make it up baffled the scientists. As it happens in these cases, new observations were made with various instruments of the Hubble Space Telescope but also with the Chandra X-ray Observatory and the ground-based telescopes Canada-France-Hawaii Telescope and Subaru Telescope on Mauna Kea.

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Those observations confirmed the presence of the dark core and scientists have proposed three theories to try to explain that anomaly.

The first possible explanation is that dark matter can interact with other dark matter and in the case of the Abell 520 cluster that interaction led to the formation of the dark core.

The second possible explanation is that the existence of the dark core is that Abell 520 is the result of the collision between three galactic clusters. That would’ve caused a more complex interaction between the various galaxies that would’ve caused the abnormal distribution of dark matter from ordinary matter.

The third possible explanation is that actually the core contains several galaxies but they emit a light too faint to be visible even by the Hubble Space Telescope. In this case, the dark core isn’t really an anomaly.

Using data obtained from the Hubble Space Telescope computer simulations will be carried out to try to reconstruct the the collision that led to the formation of the Abell 520 cluster. The results will help determine whether this anomaly is caused by insufficient data or once more the mystery of dark matter will force scientists to revise their theories.