Debate swirls about a striking regularity in the motions of spiral galaxies like the Fireworks galaxy.

Can dark matter vanquish a rival theory?

A long-smoldering feud over the existence of mysterious dark matter is heating up. For decades, a few scientists have argued that dark matter—the stuff thought to make up 85% of the matter in the universe—cannot explain a universal pattern in the motions of spiral galaxies such as our own Milky Way. Instead, they propose modifying Newton’s law of motion. Now, a leading theorist argues that dark matter can explain this pattern after all, potentially knocking the pegs out from under the rival theory, modified Newtonian dynamics (MOND).

“This is the most compelling paper I’ve seen in the context of dark matter of why there might be this kind of relation,” says James Bullock, a cosmologist at the University of California (UC), Irvine. The new paper attempts to rebut a refined case for MOND recently put forward by Stacy McGaugh, an astronomer at Case Western Reserve University in Cleveland, Ohio. But McGaugh, a leading advocate of the MOND interpretation, says theorists “have a long way to go to convince me that they’re seriously addressing the problem, let alone solving it.”

In the 1960s, astronomers including the late Vera Rubin first traced the stars circulating in spiral galaxies. They expected the stars’ speeds to slow with increasing distance from a galaxy’s heart, just as Pluto orbits the sun more slowly than Earth. Instead, the speed of the outer stars remained high, suggesting that some unseen dark matter was providing the extra gravitational force needed to keep them from flying into space. The standard model of cosmology now assumes that a galaxy forms within a vast cloud or halo of dark matter.

But dark matter has never been directly detected, and in 1983 Mordehai Milgrom, a physicist at the Weizmann Institute of Science in Rehovot, Israel, proposed instead tweaking Newton’s famous second law of motion, which says an object accelerates in proportion to the force on it. As a star circles a galaxy, the force of gravity accelerates it toward the center. Milgrom speculated that at accelerations below a low threshold, as in a galaxy’s periphery, Newton’s law changes to yield a bigger acceleration for the same force. That would enable the outer stars to circulate faster without dark matter and its extra gravity.

Many researchers dismiss MOND as ad hoc, but McGaugh has long argued that there is more to it. In November 2016 in Physical Review Letters, he and two colleagues analyzed 153 spiral galaxies and demonstrated a surprising universal trend. They found that in the galaxies, the stars’ speeds, and hence their inward accelerations, could be predicted from the distribution of stars and gas alone. That’s hard to explain with dark matter, McGaugh says, as ordinary and dark matter ought to be distributed independently. But it fits the idea that the pull of the ordinary matter—applied through MOND’s weird rule—is the only force at work. The tight relationship “just screams MOND,” he says.

However, Julio Navarro, a theorist at the University of Victoria in Canada, and colleagues say that McGaugh’s finding actually is consistent with dark matter. They note that because dark matter halos are thought to be tenuous, they alone cannot provide enough gravity to explain the motions of stars above the MOND threshold. So wherever the stars’ acceleration exceeds that level, it necessarily comes mostly from the visible matter. That explains half of McGaugh’s relation, the researchers report in a paper submitted to the Monthly Notices of the Royal Astronomical Society.

In smaller galaxies, at the low speeds and accelerations where MOND supposedly kicks in, the gravitational pull of dark matter should dominate. So one might think that the stars’ motions would be uncorrelated with ordinary matter. However, Navarro argues, models of galactic evolution show that the size of a dark matter halo correlates with that of the galaxy that forms within it. That regularity makes the total gravitational pull predictable from just the ordinary matter, he says.

Even those who find the argument persuasive say it doesn’t quite clinch the case. Navarro and colleagues plot data from four simulated galaxies, and they fall close to McGaugh’s universal curve, but not exactly on it, Bullock says. “We’re not there yet.” Others predict the discrepancies will narrow as simulations of the tricky interactions between dark and ordinary matter improve. 

Scientists already know that MOND cannot explain other phenomena that dark matter can, such as the patterns seen in the cosmic microwave background or the clustering of galaxies. So if MOND isn’t needed to explain spiral galaxies, the idea may finally fade away, says Joel Primack, a cosmologist at UC Santa Cruz: “This is its only success.”

Primack doesn’t expect McGaugh to give up the fight. Still, he says that McGaugh is often unfairly portrayed as a true believer in MOND when he is really a skeptic. “I think Stacy sees himself as the loyal opposition,” he says. “It’s his obligation to raise issues, and this is his favorite.