Stars on the outskirts of galaxies rotate unexpectedly fast. According to Newton, the farther from the center, the slower the motion should be, like in a round dance, where the outer dancers move more slowly. But in reality, stellar outskirts race along almost as fast as the core, resembling a giant carousel: on it, the outer horses fly faster so as not to fall behind the inner ones. Only a galaxy needs no engine: it just spins like that on its own.
Usually, this strange behavior is attributed to invisible dark matter, which adds unseen mass. The author of the new study took a different path: he slightly rewrote Einstein's gravity equations, linking the distribution of ordinary matter to a special kind of spacetime curvature — one that persists even far from massive bodies. Previously, the model only worked for galaxies with a uniform 'filling,' but now the scientist accounted for density variations: in the center, stars crowd together like skyscrapers, while nearer the edge, they're sparse like cottages. After calculating this for seven spiral galaxies and the Milky Way, he discovered: just a single fitting parameter, tuned using one galaxy, correctly predicts the speeds in all the others. Gravity, slightly enhanced at large distances, eliminates the need for dark particle hypotheses.
The speed of stars at the edge of the Milky Way reaches 200–250 km/s, but if only visible mass were at work, they would have to move twice as slowly.