Dark matter is the invisible and mysterious material that makes up 22 per cent of the stuff in the universe. It is one of the greatest scientific unknowns. It does not emit light; nor does it reflect light or absorb it. While we are unable to see dark matter itself, we are able to create maps of it. We can clearly pinpoint its location by observing the effects of its mass on light from distant galaxies. This can be explained with reference to Einstein, who points out that a massive object will curve the fabric of space and that light will follow this deformed path. So we can look at how light from galaxies has been bent and, consequently, infer the quantity and location of the matter that did the bending. In fact, by using this method, a team of astronomers have recently managed to create the first three-dimensional map of the immense structure of dark matter.