Explore the groundbreaking discovery of the expanding universe in this 25-minute Physics Girl video. Delve into Edwin Hubble's revolutionary work at Mount Wilson Observatory, uncovering the existence of other galaxies and the universe's expansion. Learn about the crucial role of Henrietta Swan Leavitt's variable star observations and Leavitt's Law in measuring cosmic distances. Examine Hubble's redshift observations and the speed gradient of galaxies, leading to the realization of an expanding universe. Investigate the concept of dark energy and its implications for our understanding of cosmic evolution. Gain insights into the history of astronomy and the development of modern cosmology through engaging animations and expert explanations.