Something Punched a Hole in our Galaxy, and No One Can Figure out What It Is

An unknown heavenly body has been blasting holes in our galaxy. However, this “dark impactor” is unseen. It can be speculated that this is not made of normal matter. Not even our telescopes can detect this unknown entity, but it sure is out there.

"It's a dense bullet of something," said Ana Bonaca, a researcher at the Harvard-Smithsonian Center for Astrophysics, who discovered evidence for the impactor.

Bonaca's evidence for the dark impactor, which she presented April 15 at the conference of the American Physical Society in Denver, is a series of holes in our galaxy's longest stellar stream, GD-1. Stellar streams are lines of stars moving together across galaxies, often originating in smaller blobs of stars that collided with the galaxy in question. The stars in GD-1, remnants of a "globular cluster" that plunged into the Milky Way a long time ago, are stretched out in a long line across our sky.

Under normal conditions, the stream should be more or less a single line, stretched out by our galaxy's gravity, she said in her presentation. Astronomers would expect a single gap in the stream, at the point where the original globular cluster was before its stars drifted away in two directions. But Bonaca showed that GD-1 has a second gap. And that gap has a ragged edge — a region Bonaca called GD-1's "spur" — as if something huge plunged through the stream not long ago, dragging stars in its wake with its enormous gravity. GD-1, it seems, was hit with that unseen bullet.

"We can't map [the impactor] to any luminous object that we have observed," Bonaca told Live Science. "It's much more massive than a star… Something like a million times the mass of the sun. So there are just no stars of that mass. We can rule that out. And if it were a black hole, it would be a supermassive black hole of the kind we find at the center of our own galaxy."

It's not impossible that there's a second supermassive black hole in our galaxy, Bonaca said. But we'd expect to see some sign of it, like flares or radiation from its accretion disk. And most large galaxies seem to have just a single supermassive black hole at their center.