Some stories in astronomy just keep getting rewritten, and you can actually tell how up-to-date people are by what they think the universe is up to. Take galaxies. 

Once upon a time, admittedly before I was born, folks believed everything in our universe was made of the kinds of atoms and particles that we regularly work with on Earth and experience as parts of our furniture and as unstable interlopers in our experiments. But when researchers Vera Rubin and Fritz Zwicky studied the motions of rotating galaxies and galaxies in clusters in separate research, they discovered the motions they saw didn’t match what was seen: the observed stars, gas, and dust just didn’t have enough mass to account for all the motions.

And thus, over the years, astronomers came to accept that our universe is largely constructed of material we can’t observe through light, and that stuff makes up a larger portion of our universe than the regular, scientifically named, baryonic matter. This weird stuff was given the unfortunate name dark matter, and we have been struggling to understand it ever since.

When I was in grad school, we were taught that the bulk of most galaxies is dark matter, and we even learned how to fit dark matter halos to light distributions to get at the ratio of normal to dark matter for different kinds of systems. Tiny dwarf galaxies were found with the largest dark matter ratios, and we struggled to understand what was up with ghostly, ultra-diffuse galaxies, which were just starting to be studied in earnest.

And twenty years after I finished my last grad class, researchers pointed out that at least six ultra-diffuse galaxies really didn’t seem to have dark matter. That paper, led by Pavel E. Mancera Piña, came out in 2019 and was met with a lot of “Could you look a little harder? There has got to be some dark matter there somewhere.”

So they looked harder.

And in a new paper, again led by Piña, researchers confirmed, nope, there is no dark matter to be found in at least one ultra-diffuse galaxy that they observed for forty hours using the Very Large Array (VLA) in New Mexico. Named AGC 114905, this system is about 250 million light-years away, and while it is faint and its stars are few, the VLA was able to resolve its gas and trace out how fast that gas was orbiting as a function of distance from the galactic core. That motion exactly matched what folks like Vera Rubin expected to see when they first started looking at galaxy rotation curves back before I was born, which is no longer expected. These results are the answer to the question “Can you have galaxies without dark matter?” 

Yes. The answer appears to be “Yes, you can.”

How these systems can form is still a mystery, and none of the possibilities laid out in the latest paper in the Monthly Notices of the Royal Astronomical Society are entirely satisfactory. This is one more case of the Universe showing it can be far more creative than we are and demonstrating anything is possible within the laws of physics if you mix things just so.

More Information

NOVA press release

“No need for dark matter: resolved kinematics of the ultra-diffuse galaxy AGC 114905,” Pavel E. Mancera Piña et al., to be published in Monthly Notices of the Royal Astronomical Society (preprint on arxiv.org)