Assume the universe is a big computer simulation.
A really good simulation.
If whoever created the simulation is mindful enough to prevent the intelligent and self-aware inhabitants of the simulation from realizing or even conjecturing that the universe is a simulation, then they cannot be sure that the intelligent inhabitants of the simulation are incapable of reasoning whether they live in a simulation. i.e. you cannot be sure that your simulation doesn’t realise it is a simulation.
This is a variation on the halting problem.
The universe (the one we live in) is the classic halting problem of computability.
Think about it.