This is one of the biggest hurdles of learning modern physics. We know, from experiment, that the universe is flat at large distances, and that there is not a

This is one of the biggest hurdles of learning modern physics. We know, from experiment, that the universe is flat at large distances, and that there is not a fourth dimension into which it is folded or bent. However, we also talk about "curvature" of space-time, and people's intuitive first understanding of that is that space-time somehow gets bent from the perspective of a higher number of dimensions. Curvature is very badly explained in physics, starting from the highly misleading rubber sheet analogy. If you want more pet peeves for your collection, try every explanation of gravity that shows rubber sheets, and (for advanced peeving) rubber sheet diagrams of Schwarzschild black holes that put the event horizon partway down the well, instead of where the coördinate singularity actually places it (which is infinitely far down at a finite radius).