In “Our Place in the Cosmos,” the astronomers Noam I. Libeskind and R. Brent Tully detail the discovery of Laniakea, a supercluster of about 100,000 large galaxies that includes our own Milky Way and spans nearly a half-billion light years. As detailed in this video, Laniakea’s discovery emerged from measurements of galactic positions and velocities that reveal how galaxies are moving in relation to concentrations of nearby matter and the universe’s overall expansion. In the resulting maps, previously hazy boundaries between superclusters suddenly grow sharp, delineated by swarms of galaxies confined to gargantuan gravitational basins. By studying the starry rivers and seas formed by galaxies flowing through Laniakea’s cosmic landscape, researchers hope to learn more about the properties of dark matter and dark energy.