The String Theory mentions that the entire Universe has strings as the basis of everything. It tries to make a bridge between general relativity and quantum mechanics, as they do not fit exactly in the same panel. As strange as it sound, in Theoretical Physics, some of those things add up. It is still called a theory, although it was not demonstrated practically, because we’re in the realm of Theoretical Physics.
In any case, even Einsteins Theories were not practically demonstrated until decades have passed since he described the Relativity Theories. Coma Niddy explains this and more on his science blog.