Rare galaxy found without any dark matter

Millions of light-years from Earth, there’s a galaxy that is completely devoid of dark matter — the mysterious, unseen material that is thought to permeate the Universe. Instead, the galaxy seems to be made up of just regular ol’ matter, the kind that comprises stars, planets, and dust. That makes this galaxy a rare find, and its discovery opens up new possibilities for how dark matter is distributed throughout the cosmos.

No one knows what dark matter is. True to its name, the material doesn’t emit light, so we’ve never detected it directly. All scientists know is that it’s out there based on their observations of how galaxies and stars move. Some unseen substance is affecting these deep-space objects, filling up the space between stars and clusters of galaxies. And there seems to be a lot of it. Dark matter is thought to make up 27 percent of all the mass and energy of the Universe. The matter we can see — the atoms that make up you and me — accounts for just 5 percent.

It’s not the first time that someone has claimed to have found a galaxy short in dark matter. But up until now, practically every other galaxy we’ve spotted in the Universe has had some kind of dark matter signature. In fact, dark matter is thought to be the most abundant material inside galaxies. But this new one, detailed today in Nature, has 400 times less dark matter than researchers expected to find, based on the galaxy’s size and volume. The galaxy is made up of just the mass of the star clusters inside it. There’s either a very tiny amount of dark matter — or none at all.

That means this galaxy must have formed in such a unique way that it wound up without any dark matter in it. And it raises the possibility of even more galaxies out there lacking the material, too. Perhaps there is a wide spectrum of how much dark matter a galaxy can have. “We always thought wherever you see stars, you see dark matter,” Pieter van Dokkum, an astronomy and physics professor at Yale University and the lead author of the Nature study, tells The Verge. “Maybe the only way to form a galaxy is to have dark matter, but apparently you can also form galaxies without the aid of dark matter.”

An image from NASA’s Hubble Space telescope, that indicates a ring of dark matter surrounding a galaxy cluster.
Image: NASA

Scientists have a few ideas about what dark matter might be, though nothing is certain. It could be regular matter that we just can’t see, such as neutron stars — the faint leftovers of dead suns. Or it could be brown dwarfs, the huge planet-like objects that aren’t big enough to become full-fledged stars. The problem is these objects aren’t super abundant and can’t explain all the dark matter we suspect exists. Instead, others have proposed the idea the dark matter is made up of minuscule particles we just haven’t discovered yet.

Even though we can’t see dark matter, astronomers have different tricks for detecting it. One way is to observe how it distorts light traveling through deep space. Dark matter has its own gravitational pull that can bend the light traveling from distant galaxies, making these objects appear a bit different than they actually are. Another way is to measure how fast things are moving inside a galaxy or inside galaxy clusters. If the objects appear to be moving relatively fast, it means there’s a lot more mass inside the galaxy than we can see. If things move slowly, then there isn’t as much mass. “The faster that something moves, the more mass is needed to keep it all together,” van Dokkum says, otherwise a lot of the material would shoot off into space.

This technique is how van Dokkum realized the galaxy he was observing was so strange. He and his team were using the Dragonfly Telephoto Array in New Mexico to observe big, faint galaxies that might have escaped the attention of other astronomers, perhaps because their stars are too far apart, making them difficult to see. That’s why the researchers stumbled upon this galaxy. Dubbed NGC 1052–DF2, it’s nearly the volume and overall size of the Milky Way, but it has 200 times fewer stars. And the stars it does have are clumped together in clusters that are far apart from one another.

The galaxy NGC 1052–DF2.
Image provided by Pieter van Dokkum

The researchers then measured how fast 10 of these clusters were moving and found that the stars were traveling at a very slow pace. Based on those sluggish speeds, the team figured out that the mass of the galaxy was basically the same as the mass of the stars. “It’s definitely weird, and it’s super interesting,” Jocelyn Monroe, a particle physicist and dark matter expert at Royal Holloway, University of London, who was not involved in the study, tells The Verge. “We’ve never seen one that’s almost entirely normal, luminous matter before.”

Van Dokkum and his team posed a few possibilities for how this galaxy came to be. Maybe the galaxy sprouted from a large cloud of gas that didn’t have much dark matter around it. The researchers hope to continue studying the object with the Hubble Space Telescope to get more clues about how it might have formed.

Even though its formation is a mystery, this galaxy might solve a big puzzle for astronomers. In a weird way, finding a galaxy without dark matter actually helps to strengthen the idea that dark matter exists in the first place. Some experts have argued that maybe dark matter isn’t real and that the bizarre motions of galaxies we’ve attributed to the material are actually just the laws of gravity working differently than we thought. But if that were the case, then all galaxies would behave in the same odd pattern. Finding a galaxy devoid of this behavior means that there probably is a distinct, strange substance to blame for the effects we see out in the Universe.

“For those kinds of theories, it wouldn’t be possible to ever have a galaxy that looks as though it doesn’t have dark matter,” Monroe says. “So [this galaxy is] really interesting for the potential it has to exclude some of these ideas.”

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