The Case of the Missing Dark Matter

Physicists don’t know much about dark matter. They can’t agree on what it’s made of, how much a single particle weighs, or the best way to construct a Play-Doh diorama of it. (How would you do it? Dark matter is invisible—light doesn’t interact with it at all.) Nobody has ever caught a dark matter particle on Earth.

But after 30-plus years of telescope observations, most researchers do agree on one thing: The universe contains a lot of it. Astrophysicists think dark matter dominates ordinary matter in the universe by more than five times because galaxies rotate too fast for their visible star-stuff to handle. Without the extra dark matter holding them together, the laws of physics say that these galaxies would fall apart—the Milky Way, for example, rotates so fast that it must contain 30 times more dark matter than ordinary matter. In fact, every galaxy that astronomers have ever studied contains dark matter.

Until now.

An international team of astrophysicists has discovered a galaxy 65 million light years away with so little dark matter that it may contain none at all. To arrive at this conclusion, they measured the speeds of 10 twinkly blobs in the galaxy, called globular clusters, that each contain millions of stars. Their measurements showed that this galaxy’s stars can handle its rotational speed. Compared to other galaxies of the same brightness, “it has at least 400 times less dark matter than what we expected,” says astrophysicist Pieter van Dokkum of Yale University.

This is weird—and it could change what astrophysicists think dark matter is, in addition to upending their understanding of how galaxies form, says van Dokkum. Right now, they think that galaxies form around a scaffolding made of dark matter. The stars only take shape on top of the dark matter that is already there. “Dark matter accumulates; ordinary gas falls into it; it turns into stars, and then you get a galaxy,” says astrophysicist Jeremiah Ostriker of Columbia University, who was not involved in the work.

“Finding a galaxy without dark matter is an oxymoron,” says van Dokkum. It’s like finding a body without a skeleton. “How do you form such a thing? How do you create a galaxy without dark matter first?”

However, it’s still too early to throw out the old rules, says astrophysicist James Bullock of the University of California, Irvine. He points out that the galaxy, memorably named NGC1052-DF2, is orbiting another one. It’s possible that this galaxy formed on top of dark matter just like any other, and the neighboring galaxy stripped the dark matter away, he says.

To imagine this process, you can visualize dark matter as a diffuse collection of individual particles—unlike ordinary matter, which clumps into stars and planets. “It’s better to think of it as a fluid, like a sea of dark matter,” says Bullock. The leading dark matter theory predicts that this “sea” of particles moves around a galaxy in deep, plunging orbits like comets around the sun. Bullock thinks that as the dark matter particles reached the extremes of their orbits, forces from the neighboring galaxy could have ripped them away.

The next step is to figure out whether this galaxy is an exception or the norm, says Ostriker. If astrophysicists find more similar galaxies, they’ll have to revise their current theories about dark matter. The leading theory—that dark matter consists of so-called weakly interacting massive particles, each slightly heavier than a proton—would not be able to explain the existence of many dark matter-less galaxies.

Other theories might work better. For example, Ostriker has proposed a theory that does predict certain galaxies to have extremely low amounts of dark matter, in which dark matter particles are more than 1030 times lighter than WIMPs.

If the current theory is wrong, that will also affect the strategies of the experiments trying to catch dark matter particles on Earth, says Bullock. These collaborations, such as LUX-ZEPLIN experiment in South Dakota, the XENON1T experiment in Italy, and the ADMX experiment in Washington, are trying to figure out what dark matter actually is made of, and they look to astronomical observations to guide their detector designs. LUX-ZEPLIN and XENON1T both use liquid xenon to hunt for WIMPs. ADMX looks for another candidate known as an axion, which is lighter than a WIMP and requires a different type of detector.

Van Dokkum and his team plan to keep searching for similar galaxies—or just any other weird thing that challenges the current understanding of dark matter. In 2016, they found the opposite of this galaxy—one that was rotating so fast that they concluded it was 99.99 percent dark matter. “That object was a surprise in the other direction,” he says. They don’t know how that galaxy formed, either.

They’re hoping that these weird objects will help guide theorists like Ostriker and Bullock to better understand what dark matter is. “We know so little about dark matter that any new constraint is welcome,” says van Dokkum. Even if it means throwing away what little they have.

Dark Matter

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