Why is dark matter called dark

Dark matter is "dark"

Back to the beginning: Apart from gravity, dark matter apparently has no other interaction with the universe or itself. New observations in the galaxy cluster Abell 3827 reveal that there the dark matter is not shifted compared to the four colliding galaxies. If this were also observed in other galaxy clusters, it would confirm that the dark matter particles do not even interact with themselves - contrary to previous assumptions.

Dark matter makes up a good quarter of the cosmos, but its nature is still a mystery to this day. The only thing that is clear is that their effects of gravity have had a significant impact on the structure of the universe and the development of galaxies. But it is not known which particles this form of matter, which is invisible to us and cannot be measured directly, consists of.

Interaction with yourself?

It is also controversial whether dark matter interacts through more than just gravity. "Some particle physics theories of dark matter predict additional forces that act between the particles of dark matter," explain Richard Massey of Durham University and his colleagues. The most common model, the so-called Lambda Cold Dark Matter (ΛCDM), does not provide for this.

Three years ago, Massey and his colleagues made an exciting discovery: When four galaxies collided in the Abell 3827 galaxy cluster, the dark matter seemed to be slightly shifted compared to normal matter - as if something had slowed down the dark part of these galaxies. The researchers saw this as an indication of a possible interaction between dark matter and itself.

New look at the four-man collision

But now new, more precise observations seem to refute this - at least for Abell 3827. Massey and his colleagues have examined this galaxy cluster 1.3 billion light years away this time using the Atacama Large Millimeter Array (ALMA) in Chile. In infrared light, they received a more precise picture of the galaxies and the distortion of the light of a more distant background galaxy due to the force of gravity.

923; CDM model does not interact with itself. © Andrew Robertson / Institute for Computational Cosmology, Durham University

“This distortion can be used to determine the spatial distribution of the masses of normal and dark matter in the cluster and its galaxies,” explain the astronomers. Thanks to the higher resolution of ALMA, the new images show the position of dark matter in this cluster more clearly than in the earlier observations.

But no postponement

The result: Contrary to the earlier data, this time the astronomers could not determine any shift of dark matter compared to their galaxies. Two analyzes carried out with different evaluation algorithms did not result in a statistically significant offset, as the researchers report. Instead, the distribution corresponds exactly to the pattern that it should have according to the ΛCDM model.

According to astronomers, this could mean that dark matter actually interacts through gravity alone - but it doesn't have to. It is also possible that additional force effects are either too weak to be visible in Abell 3827, as they explain. Or that the dark matter is shifted there, but exactly in our line of sight - and therefore invisible to us.

Simulation of a galaxy collision in which the dark matter (blue) does not interact with itself according to the ΛCDM model. © Andrew Robertson / Institute for Computational Cosmology, Durham University

The search continues

For astronomers, however, this result means that they are back at the beginning of their search for the nature of dark matter. "It's frustrating, but that's science: as the data becomes more accurate, the results and conclusions can change," says Massey. "As long as dark matter hardly interacts with the rest of the universe, it is difficult for us to find out what it is made of." (Monthly Notices of the Royal Astronomical Society, in press; arXiv: 1708.04245)

(Royal Astronomical Society, April 6th, 2018 - NPO)

April 6, 2018