The best map of dark matter has revealed never-before-seen structures

Scientists have created the best ever map of dark matter using subtle distortions in the shape of about 250,000 galaxies. It could help us understand some of the biggest mysteries in the cosmos.

Dark matter is extraordinarily hard to map because, true to its name, it doesn’t emit any light that we can detect. It only interacts with regular matter through its gravitational pull, so that is what researchers use to figure out where it is. Jacqueline McCleary at Northeastern University in Massachusetts and her colleagues used the James Webb Space Telescope (JWST) to do so, examining an area of sky slightly bigger than the full moon.

“It is a very high-resolution picture of the scaffolding of this little corner of the universe,” says McCleary. The resolution of the map is about twice as high as previous ones made with the Hubble Space Telescope, and it includes structures much further from Earth.

To make this map, the researchers examined the shapes of about 250,000 galaxies – but it isn’t their intrinsic shape that is interesting. “Those galaxies are basically the cosmic wallpaper,” says Liliya Williams at the University of Minnesota, who wasn’t involved in the analysis. Instead, what’s important is how the gravity of dark matter between the telescope and the “wallpaper” warps the light of the galaxies, in a process called gravitational lensing: the further the average shape of the distant galaxies is from circular, the more dark matter lies between them and us.

By analysing these differences in shape, the researchers mapped out huge clusters of galaxies, along with the filaments of the cosmic web that connects them. Some of these structures didn’t match up with anything we had previously seen while observing regular, or luminous, matter, indicating that they must be dominated by dark matter. “To identify many of these structures over a wide field, gravitational lensing is one of very, very few techniques, and definitely the best,” says Williams.

This is important because dark matter makes up about 85 per cent of the total matter in the universe, so it is crucial to the evolution of not only galaxies and galaxy clusters, but also the cosmos as a whole. Building a map of its distribution could help us nail down how it behaves and what exactly it is made of, says Williams.

“Not only is it an observational coup, but in turn it’s going to enable a lot of other analysis – cosmological parameter constraints, the connection between galaxies and their dark matter haloes and how they grow and evolve over time,” says McCleary. These cosmological parameters include the strength of dark energy, the mysterious force causing the universe to expand at an accelerating rate.

For now, it appears that the JWST map matches our current standard model of the universe, known as lambda-CDM, but there are many in-depth investigations of the data yet to be done that are certain to provide new insights, says McCleary. “Although at a glance it’s a match for lambda-CDM, I’m not giving up yet – I’m withholding judgment until our analysis is finished.”