Stars close to the centre of the galaxy orbit faster than those further out – the Sun orbits the centre in roughly 212 million years. But when astronomers measure stars near the outer edge of the galaxy, they find the motion does not slow down as much as they would predict if the distribution of the galaxy’s mass matches that of its stars and other visible material. The faster motion of outlying stars suggests that there are vast quantities of mass further out – both beyond the visible edge of the galactic disc and in the apparently empty halo region above and below. The rotation patterns of our galaxy and others are a key piece of evidence for so-called ‘dark matter’, which is thought to account for five-sixths of the mass of the entire universe and about 90 per cent of the Milky Way’s mass. Our galaxy has provided an ideal laboratory for astronomers to test theories about the nature of dark matter, allowing them to dismiss the idea that it’s simply made of conventional but hard-to-detect objects such as rogue planets, burnt-out dwarf stars or black holes in the galactic halo. Instead it seems to be a truly exotic form of matter that is not only dark, but entirely transparent and immune to interactions with any form of electromagnetic radiation, giving itself away only through its gravitational influence. Some researchers believe that the Milky Way’s dark matter halo could extend in a disc up to 2 million kilometres (1.2 million miles) across, embedded with lonely scattered stars almost halfway to our nearest large neighbouring galaxy.