String Theory
Here’s a cute video that explains string theory in the most basic terms possible.
As “beautiful” as it may be, there is no conceivable way that we can test this concept for validity, and in that way, it seems closer to a religion than a scientific theory.
This is why, IMO, it’s very possible that the human mind is incapable of understanding the fundamental building blocks of the universe. We evolved to put spears through woolly mammoths and deal with other elements of the world around us that are within a few orders of magnitude, larger or smaller, than our bodies. There is no guarantee that this brain is equipped to unify quantum physics and general relativity.