For decades, scientists have been trying to understand dark matter—the mysterious, invisible substance that makes up about 85% of all matter in the universe.
It doesn’t emit or reflect light, making it impossible to see directly, and its nature remains one of the biggest puzzles in physics.
Now, a new study from researchers at the University of Geneva (UNIGE) and their international collaborators suggests that dark matter behaves much like ordinary matter when it comes to gravity.
The findings, published in Nature Communications, indicate that dark matter falls into gravitational wells—the regions of space distorted by the pull of massive objects—just as normal matter does.
This discovery supports the idea that dark matter follows Einstein’s general theory of relativity and is not influenced by a mysterious “fifth force.”
However, the results still leave open the possibility that an undiscovered force could exist, albeit one much weaker than gravity.
Ordinary matter—the kind that makes up stars, planets, and humans—responds to four known forces: gravity, electromagnetism, and the strong and weak nuclear forces that operate inside atoms.
But dark matter, which interacts very little with anything other than gravity, has long been suspected of possibly obeying an additional, unknown force.
To test this, the UNIGE-led team examined the motion of galaxies across the universe.
Because galaxies are mostly made of dark matter, their movements should reveal how dark matter behaves under the influence of gravity.
“We compared the velocities of galaxies with the depths of the gravitational wells they fall into,” explained Camille Bonvin, a theoretical physicist at UNIGE and co-author of the study.
If dark matter followed different rules than ordinary matter, galaxies would move in unexpected ways—falling faster or slower into these wells depending on whether another force was acting on them. But the data showed no such deviations.
“Our results suggest that dark matter obeys the same equations that describe ordinary matter,” Bonvin said.
Specifically, the researchers found that Euler’s equations, which describe how matter moves under gravity, still hold true for dark matter. However, they caution that this does not fully eliminate the possibility of a new, extremely weak force. According to Nastassia Grimm, the study’s lead author, if such a fifth force exists, it would have to be less than 7% as strong as gravity, or it would have already been detected.
The team’s next goal is to push these limits even further. Future astronomical surveys—such as the Legacy Survey of Space and Time (LSST) and the Dark Energy Spectroscopic Instrument (DESI)—will collect even more precise data on galaxy motion.
These new observations could detect forces as faint as 2% of gravity’s strength, offering deeper insight into dark matter’s true nature.
“This study marks an important step forward,” said Isaac Tutusaus, co-author and researcher at the University of Toulouse. “We now know dark matter doesn’t blatantly defy gravity—but the universe may still be hiding subtle forces we’ve yet to uncover.”
