The light from early galaxies had a dramatic impact on the gases filling the universe. The stars in these galaxies gave off radiation that ionized the gas (stripping away an electron). This is a supercomputer simulation of this event, called the "epoch of reionization," which shows where the ionization due to light overwhelms that due to gravity alone. This visualization highlights the spatial structure of the early galaxies' light's effect, by comparing the ionization fraction from two simulations: one with a self-consistent radiation field (radiative), and one without (non-radiative). The yellow and red regions show where the gas has been ionized in the radiative simulation, while at the center of these blobs are small blue regions where the ionized gas from the non-radiative is concentrated. The purple illustrates the boundary at the advancing edge of the ionization, where the two simulations are the same.