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String theory is currently the largest part of theoretical physics, receiving the lion's share of both interest and funding. It has been like this since the mid-80's, when the current variations on the idea were created.
In its simplest form, string theory claims that all the observable phenomenon of the universe, including particles and fields, are the side-effects of a theoretical base item, the "string". These propagate through space-time in such a way as to minimize their surface area. That's, fundamentally, at least, it.
If that's all there is, one might ask, why is it controversial? Because, as experimental physics has advanced, a number of permutations had to be introduced into the theory. It has a large number of freely-adjustable constants that "define" the different fields, particles, etc. This makes it extremely hard to test, because if, at any time the results disagree with the current version of the theory, the string theorists can just jiggle the constants and poof, the inconsistency vanishes.
It's gotten bad enough at this point that some string theorists are beginning to push for the anthropic principle to be treated as "real" science, in order to mask the reality that string theory is so far from a testable theory that it's barely grounded in science.
An example: most consistent, finite string theories require there to be supersymmetry at the particle level; that is, every elementary particle, such as neutrons, protons, electrons, gravitons, and so on must be paired with some other particle. High-energy particle colliers, such as the ones at CERN, have yet to uncover any of these paired particles, so string theorists have just adjusted their constants to make all these unseen, hypothetical particles just outside our measurement range. If, when the new, larger collider at CERN comes online and doesn't see these particles either, string theory will just adjust their constants downwards and hide the particles from view again.