This shows a pair of galaxies chosen to be an analog of the Local Group (the Milky Way and Andromeda), here named “Romeo” and “Juliet”. This movie shows the large-scale view so you can see the full evolution of both galaxies at the same time. The simulation also happens to be the highest-resolution simulation ever of a Milky-Way mass galaxy (let alone a whole Local Group) run cosmologically, with an explicit treatment of the ISM, star formation, and stellar feedback.