There's an oft-repeated theory that our universe is a simulation -- a computer program (or equivalent) created by some higher form of intelligence. The modern popular form of this claim is generally attributed to philosopher Nick Bostrom, although the basic idea goes back much further.
And the only problem with this idea is that it is completely wrong.
The basic "simulation argument" goes like this: imagine that an intelligent race becomes intelligent enough and powerful enough that they could simulate a universe in a computer. (We already do this ourselves, in a very crude sense, when we create computer models to simulate weather or traffic or any other aspect of the real world.) Our hypothetical aliens are able to build such rich simulations, they actually contain simulated intelligent beings of their own -- and those simulated beings would (somehow) perceive themselves to be conscious.
And here's the clever twist: according to the argument, those simulated beings could become intelligent enough to build their own simulated universes, with simulated intelligent beings of their own, who in turn... Eventually, there would be an enormously large pyramid of simulations-within-simulations. And from a simple probabilistic perspective, it's enormously unlikely that we happen to be in the topmost and only real universe (and sometime in the future will ourselves start simulating universes) rather than one of the vast number of simulations.
And this is completely mistaken.
The problem with the argument is that the universe we find ourselves in is enormously complicated from the point of view of having intelligent beings in it. For a start, you could discard the other one hundred billion galaxies in our observable universe and it wouldn't make any difference to us. So it's enormously more likely that the simulated universe we are in would be much simpler than this one. (How much more likely? Borrowing an argument from Roger Penrose, possibly something of the order of 10 to the power [10 to the power 100] -- a 1 with [10 to the power 100] zeroes after it -- more likely.)
So the simulation argument turns on itself: the exact same argument that leads to the conclusion that we live in a simulation, i.e. that there are many more simulations than real universes, also inevitably leads to the conclusion that this universe isn't simulated, because there would be hugely many more simpler simulations we would be more likely to find ourselves in.