Collin A. Politsch and Rupert A.C. Croft

A majority of the atomic matter in the universe takes the form of a highly dilute gas that permeates the overwhelming volume of intergalactic space. Dubbed the intergalactic medium, this ubiquitous gas is too sparse to be observed directly, but its presence is embedded in the light of luminous background sources. Most notably, quasars—supermassive black holes that shine so brightly they can be seen billions of lightyears from Earth—allow scientists to indirectly study the structure of the intergalactic medium on vast cosmic scales.

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