Verlinde's Emergent Gravity Predicts a Key Dark Matter Pattern With No Dark Matter at All
Galaxies whose brightness differs 400,000-fold share nearly the same central dark matter surface density. A Sejong University physicist derived 170 solar masses per square parsec from gravity alone, inside the observed range.
One of the strangest regularities in astronomy is that galaxies which could hardly be more different seem to agree on one number. A new study says a theory of gravity with no dark matter in it can reproduce that number from first principles.
The regularity is called the central surface density of dark matter. Astronomers get it by multiplying a galaxy's central dark matter density by the size of its core region. Across galaxies whose brightness differs by a factor of about 400,000, from faint dwarfs to giant spirals, the value stays in a narrow band of roughly 90 to 220 solar masses per square parsec. Standard dark matter models do not naturally explain why such different objects should land in the same place.
Youngsub Yoon of Sejong University in Seoul took a different route. He started from "emergent gravity," a theory proposed in 2016 by the Dutch physicist Erik Verlinde. It treats gravity not as a fundamental force but as something that arises from the information content of spacetime, and it aims to explain galaxy rotation without invisible matter. Yoon asked what dark matter density an astronomer would infer if Verlinde's gravity were true and the astronomer assumed Newton's law plus dark matter instead. His answer was 170 solar masses per square parsec, near the middle of the observed range. The study was published Sept. 20 in the journal Physics of the Dark Universe.
The result depends on a fix Yoon proposed in 2024. In Newton's gravity, the pulls of many masses simply add up. In Verlinde's theory they do not, which made it unclear how to apply the theory to real galaxies built from many stars and gas clouds. Yoon suggested a relation to handle that, and the new paper uses it to calculate the "phantom halo" that emergent gravity would mimic. "In my 2024 study, I pointed out a technical issue in Verlinde's theory of gravity and proposed a relation to address it. It is meaningful that, in this study, the relation led to a result consistent with the actual observational value," Yoon said. "This study shows that a new theory of gravity that does not assume dark matter can explain yet another intriguing regularity observed in galaxies."
The work belongs to a decades-old line of challenges to dark matter. Since the 1970s, astronomers have known that stars in the outer parts of galaxies orbit far faster than the visible matter can explain. The mainstream answer is a halo of unseen particles. In 1983 the Israeli physicist Mordehai Milgrom proposed instead that gravity itself behaves differently when accelerations become extremely small, an idea known as MOND. Verlinde's theory reaches similar predictions from a very different starting point.
Yoon tied the result to other weak-gravity tests. He pointed to his Sejong colleague Kyu-Hyun Chae, who has reported that wide binary stars, pairs orbiting each other thousands of times farther apart than Earth and the sun, move in ways that depart from Newton's predictions at very low accelerations. Those claims are disputed, and other teams analyzing the same Gaia satellite data have found no deviation.
The result does not settle whether dark matter exists. A match with one galactic regularity is not proof, and emergent gravity still has to account for evidence on larger scales, such as galaxy cluster collisions and the pattern of peaks in the cosmic microwave background, where particle dark matter has had its strongest successes. But it adds a clean, quantitative test that any theory of gravity or dark matter now has to pass.
