There is simply no other theory that is as all-inclusive as well as having a chance of being consistent with the things we see to date. There are, however, alternatives to a quantum theory of *gravitation* (and hence of spacetime itself). The most notable one is the quantum geometry program (known in previous forms as "non-perturbative canonical quantum gravity" or "loop quantum gravity").
In summary, String Theory is certainly a rich one, with the potential of explaining in a single framework the phenomena on both small and large scales. On the downside, it is perhaps "too big", containing too many possibilities to tell us why we have the particular universe we do, and not any of the others allowed by String Theory. In addition, experimental predictions at the level of our technology are lacking.
Answered by: Sean McReynolds, M.S., Physics Doctoral Student, Penn State University
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'Natural science does not simply describe and explain nature, it is part of the interplay between nature and ourselves.'