Any matter can emit light

Self-interaction: Can dark matter do more than just gravity?

It's heavy enough to hold entire galaxies together, but remains invisible itself. It can deflect the light of distant stars, but it does not shine. What the strange dark matter consists of and what interactions it is subject to is still completely unknown. The only thing that researchers have so far found out about this crucial and difficult-to-study form of matter: Dark matter acts on normal matter exclusively through its gravity. It can neither emit or absorb light nor enter into any other non-gravitational connection with normal matter.

Nevertheless, it is one of the dominating factors in the universe: there is around five times as much dark matter as normal matter in the universe. Most galaxies can only hold their shape because they are enveloped in a large cloud of dark matter. Without them, galaxies might not even have formed.

This article is featured in Spectrum Compact, Dark Matter - The Search for the Invisible

Precise astronomical observations should help scientists to decide between the competing assumptions about the type and composition of dark matter, for example by measuring the galaxy cluster Abell 3827. This collection of elliptical galaxies is known for its large mass and is therefore particularly well suited for the analysis of dark matter Matter. Abell 3827 is around 1.4 billion light years away. Four galaxies collide there in a small space.

Measurements on a true galactic heavyweight

In recent years, astronomers have used the Gemini South Telescope to capture starlight from galaxies far behind Abell 3827. All of its matter, both normal and dark, distorts the light like a huge lens; Scientists speak of the "gravitational lensing effect". The distortion can then be used to calculate how much matter is concentrated in the area of ‚Äč‚ÄčAbell 3827. The result: the galaxy cluster contains tens of trillion solar masses, making it one of the largest known galactic heavyweights - around 50 times heavier than the Milky Way.

With such an accumulation of normal and dark matter, it could be worthwhile to look for unusual behavior once in a while: Perhaps the dark matter is subject to forces other than gravity after all? If so, it would probably interact with itself and be slowed down or deflected in the event of a collision - just like electrons or other charged particles that fly towards each other and deflect to one side due to the electromagnetic force.

Something has slowed down dark matter

The researchers were lucky with one of the four galaxies: It was so precisely in front of another galaxy in the background that the team was able to determine the distribution of visible, and especially dark, matter with great precision. To do this, they had to distinguish the position of the center of mass of this galaxy - where most of the dark matter is located - and the position of the bright stars made of normal matter.