Illustration showing the reionization of the Universe. Following the Big Bang, the Universe was little more than a soup of charged subatomic particles and energy (top, red). As the Universe expanded and cooled, there came a point ??at an age of 300, 000 years ??when most of the protons and electrons had combined to form a sea of neutral hydrogen gas. This heralded the so-called Dark Ages, when the Universe became opaque. A few hundred million years later, some of the hydrogen had started to gather into stars and, in turn, galaxies and quasars. These new structures unleashed a torrent of ionizing radiation that began to ionize the remaining neutral hydrogen, beginning the so-called Epoch of Reionization. At an age of 1 billion years, the Universe became transparent again (bottom).