When the Milky Way Flipped: How a Head-On Collision Turned Our Galaxy Upside Down
The Milky Way did not always look the way it does today. About 10 billion years ago, a cataclysmic head-on collision with a dwarf galaxy called Gaia-Sausage-Enceladus may have literally flipped our galaxy's disk by more than 90 degrees — turning it upside down before it slowly settled back into the familiar spiral we see from Earth.
1. The clue was hiding in the stellar halo — Surrounding the Milky Way's flat spiral disk is a much larger, sparser region called the stellar halo, made mostly of stars pulled in from smaller galaxies through mergers. Observations from ESA's Gaia mission showed that this halo rotates very slowly, but astronomers could not explain why. A team from Durham University used the Auriga suite of cosmological supercomputer simulations to track the evolution of 25 Milky Way-like galaxies over billions of years, looking for what made some haloes rotate slower than others.
2. The telltale signs: a slow halo and a flipped disk — The simulations revealed that galaxies with the slowest-rotating stellar haloes shared two features: they had experienced a major head-on merger with another galaxy, and they had undergone a dramatic "disk flip" — a reorientation of the galactic disk by more than 90 degrees. The Milky Way already carries the scars of just such a collision: the Gaia-Sausage-Enceladus dwarf galaxy, which crashed into the young Milky Way about 10 to 11 billion years ago and was absorbed, leaving billions of stars in highly elongated, sausage-shaped orbits.
3. What a flipped disk tells us about dark matter — Identifying a past disk flip gives astronomers a new way to understand how the Milky Way assembled over cosmic time. It also provides indirect clues about the motion of the galaxy's invisible dark matter halo. Because the disk flip would have disturbed the dark matter distribution as well, studying the aftermath helps researchers test models of how dark matter behaves — one of the biggest unanswered questions in physics. The findings were presented at the Royal Astronomical Society's National Astronomy Meeting on July 21, 2026.