The cosmic dark ages began 380,000 years after the Big Bang. At first, no stars or galaxies existed to emit light. But even after these objects began forming, their light remained shrouded because the universe was filled with a “fog” of neutral hydrogen atoms that absorb and scatter ultraviolet light. Over time, energetic ultraviolet light from early objects ionized these hydrogen atoms, knocking away their electrons. This era of reionization ended 1 billion years after the Big Bang, leaving the universe transparent to light. But astronomers aren’t sure exactly what types of objects — galaxies, black holes, or stars — were responsible for clearing the fog during reionization, says Vithal Tilvi of Arizona State University, who is part of the Cosmic Deep And Wide Narrowband (Cosmic DAWN) survey seeking to understand this era. Astronomers also aren’t sure how fast the transition from an opaque to a transparent universe occurred.