Members of the University of Geneva (UNIGE) team set out to ascertain if dark matter acts like regular matter on a cosmological scale or whether additional forces are at work. Their research, which was published in Nature Communications, raises the prospect of an as-yet-unknown interaction while suggesting similar behavior.
Map of the distribution of galaxies observed by the DESI collaboration, from which it is possible to accurately measure the velocities of galaxies. Image Credit: Claire Lamman/DESI collaboration. custom colormap package by cmastro
Does dark matter have the same laws as conventional matter? The enigma of this unseen and hypothetical component of our Universe, which neither produces nor reflects light, remains unexplained. This discovery gives some insight into the qualities of this mysterious matter, which is five times more prevalent than ordinary matter.
Ordinary matter is subject to four well-known forces: gravity, electromagnetism, and the strong and weak forces at the atomic level. So what about dark matter? Invisible and elusive, it might be subject to the same rules or guided by a fifth, unknown power.
To solve this mystery, a team led by UNIGE set out to see if, on a cosmic scale, dark matter falls into gravitational wells in the same manner as conventional matter. Massive celestial bodies cause distortions in the space occupied by our Universe, resulting in wells. Ordinary matter, including planets, stars, and galaxies, falls into these wells in accordance with well-established physical principles such as Einstein's theory of general relativity and Euler's equations. But what about dark matter?
“Dark matter falls into gravitational wells in the same way as ordinary matter, thus obeying Euler's equations,” said Authors.
To solve this enigma, a team spearheaded by UNIGE embarked on a mission to ascertain whether, on a cosmic level, dark matter behaves similarly to ordinary matter by falling into gravitational wells. The presence of massive celestial entities distorts the fabric of space within our Universe, resulting in the formation of wells. Ordinary matter, such as planets, stars, and galaxies, succumbs to these wells in accordance with established physical principles, including Einstein's theory of general relativity and Euler's equations.
To answer this question, we compared the velocities of galaxies across the Universe with the depth of gravitational wells. 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.
Camille Bonvin, Study Co-Author and Associate Professor, Department of Theoretical Physics, Faculty of Science, University of Geneva
Bonvin added, “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.”
Euler's Equations Still Valid
Using this technique with existing cosmological data, the study team determined that dark matter falls into gravitational wells in the same manner as conventional matter does, therefore following 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.
Nastassia Grimm, Study First Author, Institute of Cosmology and Gravitation, University of Portsmouth
These preliminary findings represent a significant step forward in characterizing dark matter. The next challenge will be to ascertain 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 behavior of dark matter.
Isaac Tutusaus, Study Co-Author and Associate Professor, IRAP, Midi-Pyrénées Observatory, University of Toulouse
Journal Reference:
Grimm, N., et al. (2025) Comparing the motion of dark matter and standard model particles on cosmological scales. Nature Communications. doi.org/10.1038/s41467-025-65100-8.