Imagine the universe in its infancy. Most scientists think space and time originated with the big bang. From that hot and dense start the cosmos expanded and cooled, but it took a while for stars and galaxies to start dotting the sky. It was not until about 380,000 years after the big bang that atoms could hold together and fill the universe with mostly hydrogen gas. When the cosmos was a few hundred million years old, this gas coalesced into the earliest stars, which formed in clusters that clumped together into galaxies, the oldest of which appeared 400 million years after the universe was born. To their surprise, scientists have found that another class of astronomical objects began to appear at this point, too: quasars.