A cosmic void is a vast, nearly empty region between the filaments and walls of galaxies that make up the large-scale structure of the universe. Voids can span hundreds of millions of light-years and contain far fewer galaxies than average, though they are not entirely empty. They form as gravity drains matter out of underdense regions and into the surrounding cosmic web, and they expand faster than the universe as a whole. Because dark energy dominates where matter is scarce, voids are unusually sensitive laboratories for studying it.