Einstein never actually did achieve a theory of everything. He worked toward one all of his life. The Kaluza-Klein theory only unites Einstein's field equations with those of Maxwell, or the gravitational force and the electromagnetic force. It does not describe, for example, what makes the stars shine or radioactive decay and Einstein was aware of this. Physicists have since named these forces, respectively, the strong and the weak. So we end up with four forces. EM, the strong force, and the weak have been succesfully united by quantum mechanics, a science which Einstein ironically chose not to accept even though he helped create it. QM makes use of Einstein's SR theory but cannot succesfully incorporate gravity. String theory seems to succesfully unite all four of these forces. It predicts the existence of a particle (string, not a zero-size particle) called a graviton. Scientists are still looking for evidence of the graviton, and providing that evidence is the goal of super colliders like the LHC.