A team of astronomers led by George Becker at the University of California, Riverside, has made a surprising discovery: 12.5 billion years ago, the most opaque place in the universe contained relatively little matter. It has long been known that the universe is filled with a web-like network of dark matter and gas. This “cosmic web” accounts for most of the matter in the universe, whereas galaxies like our own Milky Way make up only a small fraction. Today, the gas between galaxies is almost totally transparent because it is kept ionized — electrons detached from their atoms — by an energetic bath of ultraviolet radiation. Over a decade ago, astronomers noticed that in the very distant past — roughly 12.5 billion years ago, or about 1 billion years after the Big Bang — the gas in deep space was not only highly opaque to ultraviolet light, but its transparency varied widely from place to place, obscuring much of the light emitted by distant galaxies. Then a few years ago, a team led by Becker, then at the University of Cambridge, found that these differences in opacity were so large that either the amount of gas itself, or more likely the radia...