There's a massive hole in the outskirts of the Milky Way, and no one knows what caused it—but one scientist has an idea. "The on-sky morphology suggests a recent, close encounter with a massive and dense perturber," says Harvard scientist Ana Bonaca in a new presentation, per Newsweek. "It's a dense bullet of something." That something ripped through a stream of stars called GD-1 that moves across our sky and likely entered the galaxy a long time ago. Whatever hit them must have been about a million times more massive than a star, but stars that big don't exist. It could have been a supermassive black hole, but none are known around there. And no other large luminous objects have been spotted nearby.
Which leaves another possible culprit: dark matter. The mysterious substance, which constitutes roughly 27% of the universe, is invisible to the naked eye but has a gravitational effect on normal matter. It's also said to be "clumpy," meaning it's unevenly distributed in chunks across galaxies, like the luminous matter seen in nebulae and stars. Observations of the GD-1 hole—which is about 30 to 65 light-years across—"permit a low-mass dark-matter subhalo as a plausible candidate,” says Bonaca. Now Bonaca wants to map other parts of the sky to see what might be causing damage out there, per LiveScience. Maybe, she says, she can map dark-matter clumps all across the galaxy. (Meanwhile, there's a kind of alien presence in the Milky Way.)