The Milky Way has a cataclysmic collision in its past, made all the more mysterious because astronomers aren’t sure what caused it.

Whatever it was punched a hole straight through the galaxy, according to new research recently presented by Harvard scientist Ana Bonaca. She discovered the anomaly while analyzing scans of the Milky Way, specifically the tidal streams that renegade stars produce in their wake.

Typically, though, those tidal streams don’t have holes blasted through them. So when Bonaca noticed that one in the Milky Way did, she took a harder look. She deduced that somewhere between 6 and 10 billion years ago, a “dense bullet of something” collided with the Milky Way, tore the hole and may have caused changes in the galaxy’s star formations along the way.

That’s the million-dollar question. But there aren’t many clues to help answer it. The mysterious hole-puncher hasn’t showed up on any of our telescopes. In order to figure out what the “dense bullet” might be, astronomers can start by ruling out what it isn’t.

It wasn’t a star, Bonaca said. Why not? This hole is giant, so whatever caused it had to have been giant. As in, a million times the mass of the sun – way bigger than any stars. A supermassive black hole would have the necessary power. But it probably would have also given us other signals that it was lurking out there, so Bonaca ruled that out, too.

It just might be dark matter, one of the most mysterious substances in all of our universe. Scientists are pretty sure that dark matter exists, but it’s hard to say for sure. That’s because, well, as the name implies, it’s totally dark, meaning it doesn’t reflect light. No light, no seeing it, no proving it exists.

Still, scientists are pretty sure it’s there, because something must be. There is evidence that some kind of invisible matter with a gravitational force that plays a part in the way the galaxies rotate. It likely makes up about 27 percent of the universe.

Is it the matter that collided with the Milky Way? Maybe! It would be pretty remarkable if scientists figure out that it was, since it would probably give us more clues about what dark matter is and isn’t.

We may never know, but that isn’t going to stop Bonaca from trying to figure it out. She plans to keep studying 3-D maps of the galaxy, trying to identify more areas where this may have happened to potentially find clumps of dark matter scattered throughout the skies.