They make a point of saying wormholes aren't a naturally-occurring phenomenon, and that's the real science. Wormholes are allowed in general relativity, but large ones don't form spontaneously, and any that might exist will have a finite (and normally rather short) lifetime until the connection "pinches off" and both sides collapse into black holes.

The statement in the movie is that the wormhole was built by the five-dimensional beings, and at the end of the movie McConaughey guesses that those five-dimensional beings are actually humans from the incredibly distant future.

Now, this sets up a bit of a paradox: if the humans of the distant future were the ones who built the wormhole, and the wormhole was needed for humanity to survive into that distant future, then how did they survive long enough to build it? The answer is that the humans of the distant future built it, and they survived long enough to build it because it was there, and it was there because they survived long enough to build it, and they survived long enough to build it because it was there, ...

It's intentionally paradoxical, but this is how time works in general relativity (as far as we know). If every moment you march toward the future, you can eventually come out in what you would have ordinarily called the past. Or if you trace history long enough into the past, then you might eventually land in what you would have ordinarily called the future. The technical term for this is a "closed timelike curve", and they're a topic of much discussion in theoretical physics. I'm sure that this one was inserted intentionally into Interstellar because one of the people who founded the study of these closed timelike curves was Kip Thorne. (I linked to a paper in post 146, above.)