Newshot 
“But the question is: Is that what happened?” he continued. “And I’m not convinced that that’s the only way that can happen.” Without definitively knowing their origin story, McGaugh doesn’t think it’s possible to say anything about the nature of dark matter itself.
Most experts, though, cannot see how DF2, DF4, and DF9 could exist without dark matter being a physical thing, even if the trio’s genesis can be debated. “It’s hard to explain how you’d get something like this evolving by itself without something like dark matter,” Ghag said.
That makes this galactic trio bad news for MOND, or modified Newtonian dynamics, an alternative to dark matter proposed in 1983 by the physicist Mordehai Milgrom. “MOND says: What if there’s only visible matter, but its gravity works differently than we thought?” Whiteson explained.
MOND implies that, when objects are experiencing a very weak gravitational pull—for example, when two stars are separated by extreme distances—the gravitational force doesn’t dwindle as much as scientists think. That could explain why spinning galaxies can be held together without needing an extra source of gravity from dark matter, its proponents have argued.
Dark matter has long been the favored theory for cosmologists, as MOND “could never really explain everything,” Ghag said. He explained that MOND suffered a significant blow in 2015 when the US-based Laser Interferometer Gravitational-Wave Observatory, or LIGO, detected gravitational waves—ripples in the fabric of the universe—for the first time. Scientists found that these waves move at the speed of light in all directions over huge distances, which implies that gravity works in a consistent, unmodified way throughout the cosmos.
DF2, DF4, and DF9 are also more elegantly explained by dark matter than by attempts to adjust the mechanics of gravity. “It’s easier to remove matter, cold dark matter, through something like a collision, than it is to turn off a law of nature,” Balaji said.
“I think it’s fine and healthy to have alternative theories and open debate,” Whiteson said. “But let’s also be frank about whether the theory fits the data. MOND does not.”
The identity of dark matter still eludes us. But these galaxies are arguably another nail in the coffin of its increasingly unpopular competitor.
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.