Crude synopsis: if it is possible to create such an simulation then the probability that we in one is the number of possible simulated universes (v. large) vs the number of real universes (1) ie high probability that we are within a simulation.
I've always heard that expressed as the Simulation Paradox: either full-fidelity simulations, a la The Matrix, are impossible, or we're already in one.
Crude synopsis: if it is possible to create such an simulation then the probability that we in one is the number of possible simulated universes (v. large) vs the number of real universes (1) ie high probability that we are within a simulation.