[...]
Philosopher Nick Bostrom, director of the Future of Humanity Institute at Oxford University, describes a fake universe as a "richly detailed software simulation of people, including their historical predecessors, by a very technologically advanced civilization."
It's like the movie "The Matrix," Bostrom said, except that "instead of having brains in vats that are fed by sensory inputs from a simulator, the brains themselves would also be part of the simulation. It would be one big computer program simulating everything, including human brains down to neurons and synapses."
Bostrum is not saying that humanity is living in such a simulation. Rather, his "Simulation Argument" seeks to show that one of three possible scenarios must be true (assuming there are other intelligent civilizations):
1. All civilizations become extinct before becoming technologically mature;
2. All technologically mature civilizations lose interest in creating simulations;
3. Humanity is literally living in a computer simulation.
[…]
If people are in a whole-world simulation, how could they know it? Brin suggests a "back door" in the simulation program that would enable the alleged programmers to control people (much like countries accuse each other of installing "back doors" in code to conduct espionage).
"If we are living in a simulation, then everything is software, including every atom in our bodies," Brin said, "and there may be 'back doors' that the programmers left ajar."
https://www.space.com/30124-is-our-universe-a-fake.html