We can model the motions of planets in the Solar System quite accurately using Newton's laws of physics. But in the early 1970s, scientists noticed that this didn't work for disc galaxies - stars at their outer edges, far from the gravitational force of all the matter at their centre - were moving much faster than Newton's theory predicted.

The Milky Way hasn't been kind to the Sagittarius dwarf galaxy. Located some 70,000 light-years away, the bundle of stars has been shredded and stretched into a filamentous stream by the gravity of the Milky Way.