An N-body simulation is a computer simulation that involves a large number of particles interacting according to basic physical laws. N-body simulations are used to study how the structures in our universe may have evolved. Typically, many millions of particles are configured in an initial density distribution and allowed to interact according to the laws of gravity. The computer calculates how the particles will move under the influence of gravity in a small time step, and uses the resulting distribution of particles as the starting point for a new calculation. By calculating many time steps, the simulation can track the growth of structures in the model system. Depending on the initial density distribution and cosmological parameters selected, different structures appear at different stages of evolution. N-body simulations have provided strong support to the idea that our universe consists primarily of dark energy and dark matter. These simulations are resource intensive because the number of interactions the computer must calculate at each time step is proportional to the number of particles squared. A sophisticated N-body simulation can require tens of thousands of supercomputer hours.