Dark Matter Acts Surprisingly Normal in a New Cosmic Test
Does dark matter follow the same physical laws that govern ordinary matter? This question remains one of modern cosmology’s greatest mysteries. Dark matter does not emit, reflect, or absorb light, making it extremely difficult to observe directly. Yet it is thought to be five times more abundant than the matter that makes up stars, planets, and people.
A new study led by researchers from the University of Geneva (UNIGE), published in Nature Communications, suggests that dark matter behaves much like ordinary matter on the largest cosmic scales. While the team cannot yet rule out the presence of an additional, previously unknown force, their results significantly narrow down how different dark matter could be from the matter we know.
Do the Same Forces Act on Dark Matter?
Ordinary matter interacts through four fundamental forces: gravity, electromagnetism, the strong force, and the weak force. Dark matter, however, remains mysterious. It could obey the same forces — or it could be influenced by a subtle “fifth force” that has yet to be discovered.
Understanding which of these possibilities is correct would reshape our view of how the Universe evolves.
Testing How Dark Matter Moves Through Gravitational Wells
To investigate this, the research team asked a key question: Does dark matter move into gravitational wells the same way ordinary matter does?
Massive objects bend spacetime, creating gravitational wells. Ordinary matter falls into these wells following the principles of Einstein’s general relativity and Euler’s equations. If dark matter experienced an additional force, it would move differently.
“We compared the velocities of galaxies across the Universe with the depth of their gravitational wells,” explains Camille Bonvin, associate professor in UNIGE’s Department of Theoretical Physics. “If dark matter feels only gravity, galaxies — being mostly dark matter — should fall in exactly as ordinary matter does. A fifth force would change that motion.”
Dark Matter Follows Euler’s Equations
Using modern cosmological data, the team found that dark matter appears to flow into gravitational wells just like ordinary matter. In other words, it conforms to Euler’s equations.
Yet the story isn’t over.
“This doesn’t completely eliminate the possibility of an unknown force,” says first author Nastassia Grimm. “But if a fifth force exists, it must be weaker than 7% of gravity, or we would have already seen its effects.”
What’s Next in the Search for New Physics?
These results tighten the constraints on how exotic dark matter can be. The next step is to probe even weaker possible forces.