Only 5% of the matter that can be observed with light in space is visible, and 27%, which is five times that amount, is known as dark matter that does not emit light and pulls objects. If so, does dark matter follow the same laws of mechanics as ordinary matter, or is it subject to a new force not yet discovered?
Scientists have offered the first clear answer to this question. A team from the University of Geneva in Switzerland and the Institute of Space Sciences in Spain said on the 3rd (local time) in the international journal Nature Communications that dark matter follows the same laws of motion as ordinary matter.
The forces that govern the universe are four: gravity, the strong force, the weak force, and electromagnetism. The strong nuclear force, that is, the strong force, acts between particles such as protons and neutrons that make up the atomic nucleus and is the strongest of the four fundamental forces.
Next is electromagnetism, which acts between objects with electric charge, and third is the weak force, which appears in nuclear decay and acts over short distances. Gravity, which acts between two objects with mass, is the weakest.
Until now, academia has viewed dark matter as being governed by gravity like ordinary matter. But there were also claims that, in addition to gravity, an unknown fifth force could be at work. It has not been easy to test each claim, because dark matter cannot be seen directly.
The team instead used the speed of galaxies as an indicator. Because galaxies are mostly made of dark matter, where they move and how fast is dictated by dark matter. The researchers analyzed how galaxies move when they fall into a "gravitational well," where an extremely massive object warps spacetime.
The team examined a period 3 billion to 7 billion years ago, when the universe was much younger. Reconstructing galaxy speeds and gravitational well values in this interval to analyze the rules of dark matter motion, they found that dark matter, like ordinary matter, followed exactly the paths predicted by the Euler equations. The Euler equations describe the motion of a fluid under external forces such as gravity without considering viscosity. In other words, dark matter moved under gravity alone.
The researchers also proceeded with an analysis assuming that a fifth force exists. As a result, they concluded that even if such a force exists, its influence would be very weak. If it acts in the same direction as gravity, it would be at most 7% of gravity; if it acts in the opposite direction, at most 21% of gravity.
The team said, "If it were stronger than that, it would already have shown up in galaxy motions," while adding, "But the absence of evidence does not mean it does not exist," leaving room for possibility.
The researchers said more precise observations will be possible in next-generation observation programs such as LSST (Legacy Survey of Space and Time) at the Vera Rubin Observatory and the U.S. DESI (Dark Energy Spectroscopic Instrument). LSST repeatedly images wide swaths of the sky to sensitively capture changes in galaxy positions and brightness. DESI precisely measures distances and speeds to galaxies through spectral analysis. When these instruments go into full operation, the team expects they will be able to detect a fifth force even at the 2% level of gravity.
References
Nature Communications (2025), DOI: https://doi.org/10.1038/s41467-025-65100-8