The Milky Way Once Flipped on Its Side: New Evidence from Durham University Explains a 10-Billion-Year-Old Cosmic Mystery

For over a decade, astronomers have been puzzled by a curious observation: the Milky Way's stellar halo — a sparse, spherical cloud of ancient stars enveloping our galaxy's bright disk — rotates far more slowly than predicted. Now, a team from Durham University has presented compelling evidence that this slow spin is a relic of a violent, ancient collision that literally flipped our galaxy on its side.

The findings, presented at the Royal Astronomical Society's National Astronomy Meeting 2026, combine cutting-edge supercomputer simulations with data from the European Space Agency's Gaia mission. They paint a dramatic picture of the Milky Way's turbulent youth and offer new insights into the invisible dark matter halo that shapes our galactic home .

The Evidence: A Head-On Collision That Reoriented a Galaxy

The research reveals that the Milky Way's slow-spinning stellar halo traces back to a cataclysmic event about 10 billion years ago. At that time, a dwarf galaxy known as Gaia-Enceladus-Sausage (sometimes called the "Gaia Sausage") smashed directly into our young Milky Way. The impact was not a gentle sideswipe but a violent, head-on collision .

To understand the aftermath, the Durham team, led by astronomer Kirill Batrakov, ran a suite of supercomputer simulations using the Auriga project — a set of high-resolution models that track the formation of Milky Way-like galaxies across billions of years .

The simulations showed that head-on collisions of this type can trigger a dramatic "disc flip." The Milky Way's entire stellar disk, the bright, rotating plane of stars and gas we see from Earth as a band of light, was violently reoriented by more than 90 degrees relative to its original plane. The flip did not happen instantly but gradually over billions of years, as the disk slowly tipped over .

Solving the Stellar Halo Mystery

This ancient flip provides a clean explanation for the stellar halo's anomalously slow rotation. As the massive disk rotated and twisted, it dragged against the much larger, stable, and roughly spherical dark matter halo that envelops the entire galaxy. The dark matter halo remained fixed in its original orientation, creating friction that transferred angular momentum away from the stars in the halo .

The stellar halo — composed of ancient stars that were likely pulled in from shredded satellite galaxies — retained a "memory" of this interaction. Because it was twisted and torqued against the dark matter scaffold, it now rotates much more slowly than the disk itself, solving a long-standing puzzle in galactic astronomy .

"The project started as an investigation of the rotational velocity of the stellar halo," Batrakov told ScienceAlert, noting that the simulations revealed a direct link between galaxies that underwent major early mergers and those that ended up with slowly rotating stellar halos .

What This Means for the Solar System and the Sun's Path

The implications for our own solar system are fascinating but not alarming. The research suggests that the Sun — and the entire solar system — originally moved through the galaxy on a different orbital plane. The disk flip was so all-encompassing that it reset the trajectories of most of the galaxy's stars .

However, there is no cause for concern. The collision and subsequent reorientation happened roughly 5 to 6 billion years before the Sun and Earth even formed. The event did not destabilize the solar system because it occurred long before our home system existed. The findings simply mean that the Sun's current orbit around the galactic center is not the same path it would have followed if the flip had never occurred .

A New Probe of Dark Matter

Perhaps the most significant implication of this work is what it reveals about dark matter. While the luminous stellar disk flipped dramatically, the much more massive dark matter halo surrounding the Milky Way stayed fixed in its original orientation. The entire reorientation of the visible galaxy happened within this stable, invisible scaffold .

The slow spin of the stellar halo is interpreted as a direct signature of the gravitational torque that the disk exerted against the dark matter halo. This offers astronomers a new method to study the shape, stability, and distribution of dark matter — a substance that does not emit, absorb, or reflect light and can only be studied through its gravitational influence .

The finding aligns with other recent research that views the Milky Way's tilted stellar halo as a probe of dark matter halo geometry . By measuring precisely how the stellar halo is "tilted" and slowed, astronomers can infer properties of the dark matter distribution that would otherwise remain invisible .

The Bigger Picture

This new research adds to a growing body of evidence that the Milky Way's 13.6-billion-year history has been far more turbulent than it appears today from our serene vantage point within one of its spiral arms. The Gaia-Enceladus-Sausage collision, already known to have contributed stars and debris to the galactic bulge and inner halo, now appears to have been a defining, galaxy-altering event .

In short, the Durham team's work presents the most compelling explanation yet for why the Milky Way's giant halo of ancient stars spins so slowly. The culprit was not a slow, gradual process but a violent, transformative event in the galaxy's early adolescence — a cosmic collision that changed the face of our galactic home forever.