As you've probably suspected all along, there's a slim but real possibility that the entire universe is just one big simulation being run on the computers of hyper-intelligent pan-dimensional beings. Seriously. This is coming from scientists, people! The good news: there may be a way that we can find out.
The theory behind this whole "universe as a simulation," uh, theory, is actually not that hard to understand, even for us normal people. It goes like this: right now, physicists are capable of simulating the entirety of one infinitesimally small part of the universe. They need a supercomputer to do it, and the simulation is only of an area a few femtometers across, which is really freakin' small. But, the simulation is of everything: particles, energy, space, time, the lot. This means that, fundamentally, the simulation cannot be distinguished from the real thing: if you measure the simulation, you'd get all the same results as if you were measuring a piece of real space the same size.
Now remember, this is stuff we can already do. And it's not hard to imagine that, in the future as computers get faster and more powerful, the amount of universe that we can simulate will get a little bigger. If we can upgrade from a femtometer simulation to a micrometer simulation, say, an area of fake universe large enough to simulate a single cell. If (or when) we do that, the behavior of that cell and any measurements we make on it would be completely indistinguishable from that of a real cell. If they aren't, then the simulation needs more work, but there's no reason why (with enough computing power) this wouldn't be possible.
So now pretend that you're a hyper-intelligent pan-dimensional being equipped with the sort of computer that hyper-intelligent pan-dimensional beings tend to have lying around. This computer works just like the computer that kinda-intelligent less-pan-dimensional humans are using to simulate a femtometer-sized region of the universe, except that, instead, it can simulate the whole damn thing. All of it. And that simulation would be impossible to distinguish from something "real."
Or, nearly impossible, anyway. There is a way to tell. Find out more here...