Back in the early 1930s, astronomer Fritz Zwicky discovered a problem. Zwicky studied galaxy clusters, which can contain hundreds to thousands of galaxies loosely bound together by gravity. While examining one such cluster, he realized that the visible material within the galaxies did not have enough mass to hold the cluster together. As a result, he inferred that some dark, unseen matter must exist. Decades later, Ramanath Cowsik theorized about the source of this extra gravitational force. Cowsik, who now directs the McDonnell Center for the Space Sciences, describes the history of dark matter and shares how his discovery changed the way scientists think about this invisible force in the universe.