Einstein's general theory of relativity is tremendously successful in describing the cosmos, from our solar system to the universe as a whole. Yet, in certain situations, the theory reaches its limits: In the interior of black holes, for instance (the image to the left shows, from the outside, the active galaxy M87, which has a black hole in its centre). Or, another example, close to the big bang, when the whole of the known universe was compressed to the size of an elementary particle. For these situations, Einstein's theoriy predicts singularities: Unphysical infinite densities, infinite curvatures. A clear sign that, here Einstein's theory fails. This is not surprising: Where such microscopic length scales and high energy densities come into play, quantum theory should play a role - the other great theory of the 20th century. Any theory that, in describing such situations, ignores quantum effects, does so at its own peril.