Astronomers use enormous computer simulations of the Universe to help understand how galaxies have evolved through cosmic time. These simulations have revolutionised our understanding of astrophysics and how the earliest galaxies formed into today’s galaxies. The largest of these simulations contains over 600 million galaxies and is over 13 billion light-years across. In this talk, I will break down the fundamentals of running a simulation that models our Universe. I will also discuss how atoms, dark matter, black holes, and stars are interconnected and what we have learnt about the Universe from simulations.