Somewhere along the way to charting the most detailed 3D map of our galaxy ever, the Gaia project hit a snag. Literally.
Something had blown a massive hole in the Milky Way. Ana Bonaca of the Harvard-Smithsonian Center for Astrophysics discovered the rift, presenting her findings at a recent meeting of the American Physical Society — but we're literally in the dark on what caused it.
Indeed, the "impactor" is undetectable by telescopes and could be made of dark matter itself.
"It's a dense bullet of something," Bonaca told LiveScience.
The edge of the galaxy is already a strange place, even when you consider the overall weirdness that is space. It's sheathed in a vast halo of hot gas that's studded with old stars and globular clusters and maybe even traces of "ghost" galaxy that predates the Milky Way.
So how does an earthling perched on a pinprick of a planet tens of millions of light-years away spot a hole in that halo? For Bonaca, the answer was blowing in the wind.
She had been studying high-precision data harvested from the Gaia spacecraft, particularly on tidal streams — star clusters being blown by gravity into streams that can stretch thousands of light-years. Unless something disrupts them, those streams tend to maintain a consistent density,
Bonaca noticed a disturbance in the force: a cosmic fist punching through a tidal stream and dragging stars in its staggering gravitational wake.
"It's much more massive than a star," she told LiveScience. "Something like a million times the mass of the sun. So there are just no stars of that mass. We can rule that out."
Which really leaves us with the explanation you probably dreaded when you first saw the headline: Lock up your Infinity Stones. Thanos is on his way.
Okay, maybe we'll cycle through a few other possible explanations before we dial up the Avengers.
"If it were a black hole," Bonaca mused. "It would be a supermassive black hole of the kind we find at the center of our own galaxy."
Now, that's an exciting and lot less doom-ridden possibility. Scientists, in fact, would feast on a body of dark matter of such proportions. Although the shadowy material may make up anywhere from 27 to 95 percent of the universe, it remains its biggest mystery.
A giant dark matter glob — yes, it may be gooey — may offer us the best opportunity for prying loose those secrets. Researchers may even be able to use tidal streams, Bonaca notes in the abstract to her presentation, to measure "the mass spectrum of dark-matter substructures and even identify individual substructures."
Dark matter may fit the bill, especially since there's no sign of any object nearby that could have ripped through a tidal stream. True to its name, dark matter doesn't reflect any light. And it's virtually "unseeable."
It only exerts gravitational force.
And in this case, it may have packed a cosmic punch.