Observing the cosmic web

Astrophysics is an exercise in expanding the known horizon, pushing the limits to observe fainter objects, more distant galaxies, and stranger things. This quest recently led to the design of clever instruments to make observations not just of galaxies but of the faint, barely detectable gas around galaxies. This circumgalactic and intergalactic gas is a prediction of the lambda cold dark matter (ΛCDM) model. In this model, primordial hydrogen created in the Big Bang collapses into sheets, which in turn collapse into filaments. Galaxies form where filaments either cross or are overdense. The gas filaments feed galaxies as they grow. However, the evidence for this web of gas has remained circumstantial. On page 97 of this issue, Umehata et al. (1) present evidence for a direct detection of light from the very brightest part of this cosmic web, surrounding and illuminated by a cluster of forming galaxies.