Why does the universe separate us

5.7 billion light years away from us, two galaxy clusters collide - and provide information about the invisible part of the universe

Colliding galaxy clusters

Santa Barbara (USA) - The collision of two galaxy clusters shows that dark matter behaves differently than ordinary, visible matter. This is proven by observations made with the Chandra X-ray satellite and the Hubble space telescope, which an international team of astronomers will soon be reporting on in the "Astrophysical Journal". While the gas in the galaxy cluster is slowed down by the collision and accumulates in the middle, the dark matter of the two clusters flows through each other almost unhindered.

It is not the first case, this 5.7 billion light-years distant double cluster with the catalog name MACS J0025.4-1222, in which the sky researchers observe such a separation of dark matter from ordinary matter. Another team of astronomers reported a similar phenomenon in the so-called "bullet cluster" two years ago. But since this still seemed to be an exceptional case, there were doubts about the correct interpretation of the observations.

Now, however, the observations of the colliding galaxy clusters MACS J0025.4-1222 by Maruša Bradaç of the University of California at Santa Barbara and her teammates show that the separation of dark and normal matter appears to be the rule when galaxy clusters collide. The astronomers were able to observe the glow of the hot gas in the clusters in the X-ray range with the Chandra satellite. To determine the distribution of dark matter, the astronomers analyzed the shape of galaxies far behind the double cluster. Due to its gravity, the MACS J0025 acts like a gigantic, distorting lens - and the strength of the distortion can be used to calculate the distribution of the mass in the colliding clusters.

The clear separation confirms that dark matter interacts with normal matter - and also with itself - almost exclusively via gravity. Dark matter makes up about 85 percent of the mass of the cosmos and consists of previously unknown elementary particles. Without dark matter, large structures such as galaxies and galaxy clusters in the universe could neither arise nor stick together - the gravity of normal matter would not be sufficient for this.