The basic idea of string theory is to create a simple, elegant mathematical framework for the basic forces in nature. Three of them had already been reduced to each other (electro-magnetism, weak nuclear force, strong nuclear force), but not gravity, which is much weaker.
String theorists started out by postulating a particle for gravity, describing it with the same formalisms as other interaction particles. This turned out to be elegant and consistent. The mathematical model was that of strings.
An experiment was now needed to see if this theory was not only consistent, but also correct. Not surprisingly, string theory predicted the existence of gravity. (Duh.) To explain why gravity was weaker than its siblings, additional dimensions had to be introduced, formulating our apparent four-dimensional manifold as a thouroughly warped and twisted Calabi-Yau-space. Thus string theorists contributed greatly to pure mathematics.
To determine what shape our continuum would have to take, and how many dimensions were needed, experiments were conducted to see at what distances gravity would become stronger. Nothing was found so far, so the number of spatial dimensions needed for string theory to remain consistent is somewhere upwards of eleven.
Meanwhile, string theory also provides an elegant possible explanation of where the Big Bang came from, introducing parallel universes. It also doubled the number of possible fundamental particles via the concept of super-symmetry. None of the newly predicted particles have been found so far.
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