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The Sciences

The X-Rays Thought to Prove What Makes Up Dark Matter Don’t, In Fact, Prove the Connection

Dark matter might still be made of sterile neutrinos — but scans of our own galaxy couldn’t find the supposed signals proving the connection.

By Leslie NemoMarch 26, 2020 7:42 PM
Milky Way Galaxy - NASA
The Milky Way, seen edge-on. (Credit: NASA)

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Despite knowing dark matter is out there, astrophysicists are still missing one critical detail. “We know where it is and how much of it there is, but we don’t know what it is,” says Ben Safdi, an astrophysicist at the University of Michigan.

Some think this mysterious substance is made of hypothetical subatomic particles called sterile neutrinos. Unlike regular neutrinos, which interact via both gravity and the weak nuclear force, sterile neutrinos would only interact via gravity. And that seems to be the exact same rules of engagement that dark matter follows.

For this reason, astrophysicists since 2014 have sought the X-ray signals that would be produced if sterile neutrinos were decaying into normal matter as hypothesized. If such X-ray signatures were found, it would strongly suggest sterile neutrinos are a (if not the) elusive source of dark matter.

But after sifting through 20 years’ worth of data from the darkest, blackest regions of our own galaxy in search of that special X-ray signal, Safdi and his research team saw nothing.

That means there is currently no experimental evidence that dark matter is made of the sterile neutrino, Safdi and his team concluded in their work, published in Science.

“We were disappointed. This is not the result we were hoping for,” Safdi says. Their results don’t mean that sterile neutrinos aren’t the source of dark matter — just that this particular X-ray will not prove the connection. So now it’s back to the drawing board to see what else might link the two together. 

Taking a Closer Look

Though other efforts to link dark matter to those signals had looked at faraway galaxies, Safdi and his co-authors decided to look closer to home.

“The leap we made was that if [the X-ray signal is issuing] there, then it should also be [issuing] around our own galaxy.” The Milky Way, where our solar system resides, is surrounded by the mysterious substance, Safdi says. The team searched the absolute darkest, blankest portions of our galaxy — parts that are presumably only dark matter — for the X-ray signal. But no such X-ray appeared in the team’s data.

It’s possible this X-ray signal appeared in other dark matter searches as a kind of contaminant, Safdi says. Looking at galaxies far away means interference from gases, stars and other cosmic materials end up in the data. Dark matter information from our own galaxy is more pure. “We were looking for dark matter radiation on top of nothing,” Safdi says.

A New Approach

Safdi and his team think that maybe other kinds of X-rays — ones with different wavelengths — might still prove that sterile neutrinos make up dark matter. They’re now applying their same research concept to these different X-rays to see if those might be radiating from the blackest regions of the Milky Way.

“We now have this powerful method we can apply to this data,” Safdi says, “And, who knows? Maybe we’ll find evidence for dark matter.”

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