Very small objects, such as atoms, are subject to the laws of quantum physics. Conversely, very large objects, for example a galaxy, follow the laws of general relativity. However, since galaxies also consist of individual atoms, there should actually be a theory that describes both the very large and the very small at the same time. However, such a theory would have to combine quantum physics and general relativity – a challenge that physics has so far failed to meet, despite 100 years of effort. It almost seems as if the properties of very large and very small objects contradict each other. In this talk we will give an insight into these conceptual difficulties and show why physicists today are optimistic that they will be able to overcome them.