Simulation theory, or the simulation hypothesis, proposes that all of reality, including the Earth and the universe, is in fact an artificial simulation, most likely a computer simulation.
The best-known formulation is the philosopher Nick Bostrom's 2003 simulation argument, a trilemma holding that at least one of three claims is almost certainly true: civilizations almost never reach technological maturity; mature civilizations almost never choose to run simulations of their ancestors; or nearly everyone with experiences like ours is living inside a simulation.
Popular accounts, such as the explainer video Is Reality Real?, reduce the case to a chain of requirements for the theory to be true. It must be possible to simulate consciousness. Technological progress must not stop anytime soon. Advanced simulations must not destroy themselves. Super-advanced civilizations must want to run simulations. If all of that holds and there are a lot of simulations, then any given observer is probably inside one. The reasoning overlaps with the Fermi paradox, since both turn on whether civilizations survive long enough to become technologically advanced.
Critics dispute the premises. Some question whether a simulated mind would be conscious at all; physicists such as Sabine Hossenfelder and George Ellis call the idea unscientific or technically impracticable; others note that simulations could nest inside simulations without end.