# Universe

Universe is everything that exists, it is the [spacetime](spacetime.md) and everything in it, all matter and energy as well as all the laws of nature by which it behaves. The size of the whole universe is not known, it is possibly [infinite](infinity.md), however the size of the observable universe -- the part of it with which we can ever interact due to limited [speed of light](speed_of_light.md) combined with constant expansion of space -- is about 93 billion [light years](light_year.md) in diameter, and contains an estimated number of 100 billion [galaxies](galaxy.md), each containing hundreds of billions of [stars](star.md). Current science says the universe is about 13.7 billion years old and that it started with the [Big Bang](big_bang.md), a point in time from which everything began to rapidly expand from a single point in space.

[Computers](computer.md) can be used to [simulate](simulation.md) certain parts of the universe, in fact all programs mimic the universe in some kind of simplified way, be it scientific simulations of planet collisions, government databases or [games](game.md) -- they all more or less accurately model the reality. We can also see computers as a way of creating smaller universes, which leads many to think our universe may itself be a [simulation](simulation_hypothesis.md) running on some computer in a "bigger" universe (note that this probably isn't testable and the debate isn't scientific, but we can lead philosophical discussions about it).