LIGO confirmed the existence of gravitational waves in 2015, detecting one-time perturbations of spacetime from the merger of large black holes. There should be a background of gravitational waves permeating space due to other cataclysmic events in the cosmos, perhaps dating from the Big Bang. The NANOGrav consortium reports evidence from pulsar timing arrays that space is filled with such waves. Simulations suggest the cosmic gravitational wave background is likely caused by the merger of hundreds of thousands of supermassive black holes over the history of the universe.