The largest galaxy simulations have been done by the Gauss Center for Supercomputing. The team used 24,000 cores to simulate the formation and interaction of galaxies. Read more about their most recent simulation here: https://www.sciencedaily.com/releases/2018/03/180319120459.htm
One of my favorite particle simulations is this video showing how a gravitationally contracting cloud forms a rotating disk due to the conservation of angular momentum. It's the reason why the solar system and spiral disk galaxies are flat, and the simulation helps give an intuitive reason why this occurs.
Really interesting to see how ideas in computer graphics is used quite a lot in understanding space. Another example (not entirely related to particle simulations) is the black hole simulation used for Interstellar: https://www.wired.com/2014/10/astrophysics-interstellar-black-hole/