Galaxy Rotation Curve: The Dark Matter Problem

Dark matter makes up roughly 27% of the universe's energy density but has never been directly detected — its existence is inferred entirely from its gravitational effects. This simulator recreates the single most famous piece of that evidence: the galaxy rotation curve. A spiral galaxy of thousands of stars orbits its center, each star's speed set by how much mass sits inside its orbit. With no dark matter, that speed should fall off with distance, the same way planets further from the Sun move slower. Turn the dark matter halo up and the outer stars keep moving fast, flattening the curve — matching what every well-measured spiral galaxy actually shows.