This image shows a computer simulation (more than 50 million light years across) of one possible scenario for the large-scale distribution of light sources in the universe. The details of how light is distributed through the cosmos is still not a settled question. In particular, the relative contributions of (faint but numerous) galaxies and (bright but rare) quasars is unknown. However astronomers know that, on the largest scales, the universe is structured as a vast web made up of filaments and clusters of gas, galaxies and dark matter, separated by huge, dark voids. Observational astronomy is making strides in mapping out these structures in gas and light, but the smallest galaxies – less than a pixel across on this image – might never be seen directly because they are simply too faint. These computer models are one way of trying to extrapolate from what we know to what’s really there; new research from UCL now shows how we can also use future observations of gas to find out more about this elusive population of tiny galaxies.