Scientists have conducted a precise test showing that dark matter follows the same physical rules as ordinary matter when moving through cosmic gravitational wells. The study, led by researchers at the University of Geneva, suggests no strong evidence for a fifth force influencing dark matter. However, a weaker unknown interaction remains possible.
Dark matter, which makes up about five times more mass in the universe than ordinary matter, has long puzzled cosmologists because it neither emits nor reflects light. A new study from the University of Geneva (UNIGE) and collaborators, published in Nature Communications in 2025, provides one of the sharpest tests yet of its behavior on cosmic scales.
The researchers investigated whether dark matter responds to the four known fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—or if an additional fifth force might affect it. To do this, they compared the velocities of galaxies, which are predominantly composed of dark matter, with the depths of gravitational wells formed by massive cosmic structures. These wells distort spacetime according to Einstein's general relativity, and ordinary matter falls into them following Euler's equations of fluid dynamics.
"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," explained Camille Bonvin, associate professor in UNIGE's Department of Theoretical Physics and co-author of the study. "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."
Analyzing modern cosmological data, the team found that dark matter's motion aligns with Euler's equations, mirroring ordinary matter. "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," said lead author Nastassia Grimm, a former UNIGE postdoctoral researcher now at the University of Portsmouth.
The findings, detailed in the paper "Comparing the motion of dark matter and standard model particles on cosmological scales" (DOI: 10.1038/s41467-025-65100-8), represent a step toward clarifying dark matter's role in universe formation. Future observations from projects like the Large Synoptic Survey Telescope (LSST) and Dark Energy Spectroscopic Instrument (DESI) could detect forces as faint as 2% of gravity, potentially revealing new physics, according to co-author Isaac Tutusaus of ICE-CSIC and the University of Toulouse.