Astronomers say for the first time, they've discovered the tell-tale sign another galaxy crashed into ours, leaving behind 'shells' of stars travelling in unexpected directions.
In 2005, scientists announced the discovery of a "rather pathetic galaxy" in the constellation of Virgo, calling it the Virgo Overdensity. It was suggested then it was actually the remains of a bigger galaxy in the process of being consumed by our own Milky Way.
Measurements showed the stars were moving in different directions - some towards us, some away - and now astronomers think they know what's going on.
"When we put it together, it was an 'aha' moment," said Heidi Jo Newberg, lead author of a new study into the phenomenon.
"This group of stars had a whole bunch of different velocities, which was very strange. But now that we see their motion as a whole, we understand why the velocities are different, and why they are moving the way that they are."
Data and simulations (as seen in the video) suggest a dwarf galaxy plunged right into the centre of ours about 2.7 billion years ago in "the stellar version of a T-bone crash" according to the Rensselaer Polytechnic Institute, where Dr Newburg works.
Rather than the Milky Way just absorbing the other galaxy's stars, the speed and momentum of the other galaxy saw them pass right through at first - before slowing down thanks to gravity.
While most were pulled back towards the Milky Way's star-heavy galactic core, some remained.
"Each time the dwarf galaxy stars pass quickly through the galaxy centre, slow down as they are pulled back by the Milky Way's gravity until they stop at their farthest point, and then turn around to crash through the centre again, another shell structure is created," Rensselaer said.
These shells have been seen in other galaxies, but not ours - until now.
"There are other galaxies, typically more spherical galaxies, that have a very pronounced shell structure, so you know that these things happen, but we've looked in the Milky Way and hadn't seen really obvious, gigantic shells," said Thomas Donlon, student at Rensselaer and co-author of the new study.
"It just looks different because, for one thing, we're inside the Milky Way, so we have a different perspective, and also this is a disk galaxy and we don't have as many examples of shell structures in disk galaxies."
Dr Newburg said it's likely most, if not all, of the stars in the Milky Way's surrounding halo are "immigrants" like those from the Virgo Overdensity, but T-bone crashes are rare.
The next major collision the Milky Way faces is with the Andromeda galaxy, in about 4.5 billion years. If the sun's still around then, there's a chance it could either be torn apart after coming too close to either of the galaxies' supermassive black holes, or flung out into intergalactic space.
The new findings were published in The Astrophysical Journal.
