Riccardo Della Monica

Something massive hides at the center of the Milky Way — four million times heavier than the Sun. The star S2, like a racer on a steep curve, loops around this object every sixteen years in an elongated orbit, accelerating to 7650 kilometers per second. By tracking exactly how it moves, astronomers are trying to figure out what that 'something' is.

A swing swings higher and higher if you push it in rhythm. A similar mechanism kicks in when a dead star orbits a massive black hole shrouded in a cloud of ultralight particles. These particles are candidates for dark matter — the invisible substance that keeps galaxies from flying apart.

At the center of a galaxy, a huge black hole is shrouded in an invisible cloud — a swarm of ultralight particles that it itself spawned. Into this cloud dives another black hole, about the size of a star, and begins a long spiral dance. The approach takes millions of years, but the most dramatic events unfold right at the horizon.