Imagine making a night-time trek to a remote stretch of desert, far from any sign of civilisation. You crest a hill and are astonished to find a building ablaze with artificial lighting.
That is a little like the puzzlement that greeted the discovery of Hanny's Voorwerp, a curious gas cloud found floating in intergalactic space in 2007. It is brighter than 30,000 suns but has no obvious power source. Now, 19 similar clouds have been discovered, all glowing apparently without internal power.