What is dark matter

Astrophysics: Six Explanations for Dark Matter

What is the universe made of? It used to be thought that it was just "normal" matter, that is, the substance of which stars, planets and we humans are made. However, the measurement of galaxy movements and the mapping of large-scale structures in the cosmos has since convinced many experts that there must be another form of matter that is invisible to the human eye and to telescopes. But what is hidden behind this ominous dark matter? Is it a previously unknown elementary particle? And if so, which one? Or is there something wrong with our understanding of gravity? We have compiled the most plausible theories.

1. Hidden ordinary matter

When astronomers discovered in the 1970s that there was a lack of matter in space, they initially assumed that not all ordinary matter had yet been found. For example, there could be very cold gas or dust clouds that do not emit infrared radiation. However, they would have to have huge dimensions and would warm up noticeably in the medium term if they were in the vicinity of a galaxy.

Chilled brown dwarfs and similar massive bodies are another possibility. Researchers affectionately call them MACHOs (Massive Astrophysical Compact Halo Objects). They also consist of normal matter, i.e. atoms and known elementary particles. However, in many cases they are too dark to show up on telescope images.

You can still track them down in a roundabout way: Brown dwarfs can act as gravitational lenses and amplify starlight when they pass in front of a star. Astronomers have rarely seen such events. Most researchers therefore agree: MACHOs do not even come close to bringing together the mass that is needed for dark matter, even with very optimistic estimates.

This probably also applies to black holes: large numbers of them could drift through space at the edge of galaxies and, thanks to their great gravitational attraction, explain the effects that astrophysicists attribute to dark matter. The dark lumps of mass are also very effective gravitational lenses. However, there cannot be too many of them out in space, as sky surveys have shown.

Nevertheless, some experts hold on to this option. Among other things, because gravitational wave detectors have detected black holes with a weight of around 50 solar masses in recent years that were not previously on the screen.