Every galaxy floats in an invisible cloud of gas, like fruit in syrup. This gas is the leftover building material from which stars were assembled, and it is constantly replenished by fresh supplies from intergalactic space. Seeing it directly is nearly impossible: it is too tenuous and too cold. But when light from a distant quasar passes through such a cloud, metal atoms in the gas absorb some of the light at specific wavelengths. From these 'dips' in the spectrum, astronomers calculate how much gas the light encountered along the way.

Scientists took one of the best computer models of the universe (IllustrisTNG, where millions of virtual galaxies live by the laws of physics) and compared its predictions with real observations. In the model, they considered two scenarios: gas heated only by particle collisions, or gas also illuminated by ultraviolet radiation from all stars and quasars. It turned out that the second scenario matches what telescopes see much better. The model describes particularly well the tenuous gas on the outskirts of galaxies: that very 'syrup' stretching for hundreds of thousands of light-years.

Why does the model fall short on dense clouds? Possibly it lacks some processes that compress gas: supernova explosions, outflows from black holes, or galaxy collisions. It's like a baker forgetting to add yeast to the dough — the pie turns out, but not as fluffy. The difference between model and reality points to which 'yeast' needs to be added to the next version of the simulation.

This approach, comparing model with observations, helps calibrate our understanding of how galaxies breathe: pulling in gas, processing it into stars, and ejecting it back out. The more accurately the model reproduces the pattern of quasar light absorption, the more confidently we can say we correctly understand the circulation of matter in the universe.