In the cold wilderness of space, galaxies huddle together around the campfires of stars and the assuring pull of supermassive black holes. Between these cozy clusters of galaxies, where empty space stretches on for millions of light-years all around, a faint highway of gas bridges the darkness. This gassy, intergalactic network is known in cosmological models as the cosmic web. Made of long filaments of hydrogen left over from the Big Bang, the web is thought to contain most (more than 60%) of the gas in the universe and to directly feed all of the star-producing regions in space. At...
