In many ways DESI is the successor to an older survey, the Sloan Digital Sky Survey. Like any dense material, there were sound waves – waves of pressure that crisscrossed the universe. Many of these sound waves were triggered by a competition between gravity and radiation. This meant that radiation could no longer play the game, and the back-and-forth sound waves got stuck mid seesaw. In fact we even have pictures of these features, which are the baryon acoustic oscillations (or “super hot sound waves” if you prefer).