A team of astronomers use cosmological data to test if dark matter behaves like ordinary matter

Does dark matter follow the same laws as ordinary matter? The mystery of this invisible and hypothetical component of our Universe—which neither emits nor reflects light—remains unsolved. A team led by the University of Geneva (UNIGE), with the participation of the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) and the Institute of Space Sciences (ICE-CSIC), delved into whether, on a cosmological scale, this matter behaves like ordinary matter or whether other forces come into play. Their findings, published in Nature Communications, suggest a similar behaviour, while leaving open the possibility of an as-yet-unknown interaction. This breakthrough sheds a little more light on the properties of dark matter, which is five times more abundant than ordinary matter.

Ordinary matter obeys four well-identified forces: gravity, electromagnetism, and the strong and weak forces at the atomic level. Invisible and elusive, dark matter could be subject to the same laws or governed by a fifth, as yet unknown force.

To unravel this mystery, the team investigated if dark matter falls into gravitational wells in the same way as ordinary matter. Under the influence of massive celestial bodies, the space occupied by our Universe is distorted, creating wells. Ordinary matter—planets, stars and galaxies—falls into these wells according to well-established physical laws, including Einstein’s theory of general relativity and Euler’s equations. But what about dark matter?

“To answer this question, we compared the velocities of galaxies across the Universe with the depth of gravitational wells,” explains Camille Bonvin, associate professor in the Department of Theoretical Physics at UNIGE’s Faculty of Science and co-author of the study. “If dark matter is not subject to a fifth force, then galaxies—which are mostly made of dark matter—will fall into these wells like ordinary matter, governed solely by gravity. On the other hand, if a fifth force acts on dark matter, it will influence the motion of galaxies, which would then fall into the wells differently. By comparing the depth of the wells with the galaxies’ velocities, we can therefore test for the presence of such a force.”

Applying this approach to current cosmological data, the research team concluded that dark matter falls into gravitational wells in the same way as ordinary matter, thus obeying Euler’s equations. “At this stage, however, these conclusions do not yet rule out the presence of an unknown force. But if such a fifth force exists, it cannot exceed 7% of the strength of gravity—otherwise it would already have appeared in our analyses,” says Nastassia Grimm, first author of the study, former postdoctoral researcher at the Department of Theoretical Physics at UNIGE’s Faculty of Science who has recently joined the Institute of Cosmology and Gravitation at the University of Portsmouth.

These initial results mark a major step forward in characterising dark matter. The next challenge will be to determine whether a fifth force governs it. “Upcoming data from the newest experiments, such as LSST and DESI, will be sensitive to forces as weak as 2% of gravity. They should therefore allow us to learn even more about the behaviour of dark matter,” concludes Isaac Tutusaus, IEEC researcher at the ICE-CSIC, associate professor at IRAP, Midi-Pyrénées observatory, University of Toulouse, and co-author of the study.

Lead Researcher at the IEEC

Isaac Tutusaus

Institute of Space Studies of Catalonia (IEEC)
Institute of Space Sciences (ICE-CSIC)
E-mail: tutusaus@ieec.cat, tutusaus@ice.csic.es