There is significant evidence that the majority of matter in the universe — roughly 85% — is not made of atoms. Whatever that matter is, it gravitates, does not interact with light the same way normal matter does, and is responsible for the formation of galaxies and most other large-scale structure in the universe. In this public lecture, recorded on April 21, 2023, David E. Kaplan presents historical context and evidence for dark matter. He describes the many attempts to find other properties of this invisible substance, including efforts to directly detect it in laboratories. Kaplan received his Ph.D. from the University of Washington in 1999. He held postdoctoral positions at the University of Chicago/Argonne National Laboratory and SLAC National Accelerator Laboratory. He joined the faculty of Johns Hopkins University in 2002. An American Physical Society Fellow, Kaplan was also awarded a National Academy of Sciences Kavli Frontiers of Science Fellowship and an Alfred P. Sloan Fellowship. He was named Outstanding Junior Investigator by the U.S. Department of Energy. Kaplan created and produced the documentary “Particle Fever” about the discovery of the Higgs particle. The film premiered in 2013 and won an Alfred I. DuPont-Columbia University Award and other accolades. For information about the Fermilab Arts and Lecture Series, please visit: https://events.fnal.gov/arts-lecture-series/