The Sagittarius galaxy plunged into the disk of the Milky Way and passed through again and again, like a stone thrown into a still pond. Each strike left ripples: waves and warps in the star field. Once, Sagittarius brimmed with invisible mass (dark matter), but the collisions stripped away its gas and stars, turning it into a faint trail.

Astronomers simulated the last major impact about a billion years ago to see if it could spawn spiral arms, a warped disk, and subtle oscillations in star velocities. Motion measurements via spectroscopy confirmed: the impact explains the arms and warp, but more complex wave patterns aren’t fully reproduced. So our Galaxy remembers other shoves — from the dense central bar or other dwarf companions.

The most surprising part: Sagittarius itself has nearly vanished, but its dark matter likely still permeates the Milky Way, invisibly swaying stellar orbits. Like cosmic detectives, scientists piece together the Galaxy’s violent past.