The Millennium Simulation, formally known as the Millennium Run, is one of the largest ever simulations of the development of the universe. The Millennium Simulation was developed in 2005 by the Virgo Consortium, a group of astrophysicists from Germany, the UK, Canada, Japan, and the USA. The simulation, which was run on a supercomputer in Garching, Germany, included over 10 billion "particles," simulating 20 million galaxies and quasars in a virtual cube about 2 billion light years on a side. The Millennium Simulation was created as a tool to produce predictions about the large-scale structure of the universe and compare them against observational data and the theories of astrophysicists.
The Millennium Simulation begins about 379,000 years after the Big Bang, 13.7 billion years ago, when the universe was extremely dense and hot. And that time, matter consisted of a plasma of electrons, photons, and baryons, and the universe was bathed in a radiation flux. As the universe expanded and cooled, it reached a critical temperature -- about 3000 K -- and began to "decouple" into radiation and independent matter. This event produced the cosmic microwave background radiation, which today saturates the universe and has a universal temperature of about 2.7 K. Because of detailed observations of the cosmic microwave background, physicists have a good idea about the state of the universe at the instant of decoupling, and this information was programmed in to the Millennium Simulation to serve as its initial state.
After running the Millennium Simulation on a powerful supercomputer for over a month, the Virgo Consortium got their results -- over 25 terabytes (TB) of data, enough to fit on 5,300 DVDs. Displayed in visual form, the output looks like a fine three-dimensional web of filaments with fractal self-similarity on multiple layers of organization. These filaments are actually dark matter, which makes up most of the mass in the universe. Dark matter cannot be seen directly, but its existence can be inferred from its gravitational influence on visible matter. In the model, the filaments can be seen directly, something impossible with real dark matter.
Running the Millennium Simulation gave astrophysicists an abundance of new data about how the universe might have evolved, and predicts the "supercluster" structure that we observe from astronomical data. One of the earliest results derived from the Millennium Simulation was that black holes were able to have formed earlier than previously thought, something supported by experimental data from the Sloan Digital Sky Survey, but which challenges our current cosmological models.