A new international study has provided one of the clearest signs yet that dark matter—not a change in gravity—is responsible for the mysterious way galaxies spin.
By studying some of the smallest and faintest galaxies in the universe, scientists have found evidence that the invisible substance known as dark matter continues to be the best explanation for how galaxies move.
The research, led by the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany, involved scientists from universities in the UK, China, Portugal, the Netherlands, and Sweden.
The team analyzed how stars move within 12 dwarf galaxies—tiny, dim galaxies that contain only a few million stars compared to the hundreds of billions found in galaxies like the Milky Way.
For decades, astronomers have known that galaxies rotate much faster than their visible matter—stars, gas, and dust—should allow.
To explain this, scientists proposed the existence of dark matter, an unseen substance that exerts gravity but doesn’t emit or reflect light.
However, some researchers have argued for a competing idea known as Modified Newtonian Dynamics, or MOND. This theory suggests that gravity behaves differently at extremely low accelerations, eliminating the need for dark matter altogether.
The new study, accepted for publication in Astronomy & Astrophysics and currently available on the arXiv preprint server, tested these competing ideas using highly detailed data.
The team measured how fast stars move at different distances from the centers of dwarf galaxies and compared the results with predictions from both dark matter and MOND models.
“The smallest dwarf galaxies have long been a challenge for MOND,” said Mariana Júlio, a Ph.D. student at AIP and the study’s lead author.
“But for the first time, we were able to precisely measure the internal gravitational forces in these galaxies. What we found doesn’t match what MOND predicts—but it does fit very well with models that include dark matter.”
Using powerful computer simulations run on the UK’s DiRAC National Supercomputer, the researchers showed that the observed motions of stars could only be explained if the galaxies were surrounded by large halos of dark matter.
In contrast, MOND failed to reproduce the data, even when accounting for uncertainties or possible variations in the theory.
Dr. Marcel Pawlowski, a co-author from AIP, explained that the findings also overturn a long-standing assumption in astronomy called the “radial acceleration relation.” This rule suggests that there’s a predictable link between how much visible matter a galaxy has and the gravitational pull it produces. “We found that this relationship breaks down in the smallest galaxies,” Pawlowski said. “They show stronger gravitational effects than their visible matter can account for—which means more missing mass.”
In some of these tiny galaxies, two systems with similar amounts of visible stars and gas can have very different gravitational forces. This inconsistency, the researchers argue, is naturally explained if dark matter is present but invisible.
Professor Justin Read from the University of Surrey, another co-author, said the results highlight the power of new data and modeling techniques. “We can now map the gravitational fields of galaxies on smaller scales than ever before,” he said. “What we’re seeing strongly suggests that something unseen—dark matter—is shaping their motion.”
While the study doesn’t solve the mystery of what dark matter actually is, it further weakens the case for modified gravity theories like MOND.
As astronomers continue to study even smaller and more distant galaxies, they hope to move closer to understanding the true nature of the universe’s hidden mass.
