SCO, Murray Hill
One problem with string theory is that it only works in ten or twenty-six dimensions. This is because if there are any other number of dimensions, mathematical anomalies appear. The question then has to be asked, where are the other six dimensions? In normal life there are only four. However, the Kaluza-Klein Theory shows that it is possible for a dimension to be "curled" up into an extremely tiny ball (10-31 cm long), which we could obviously not detect. In string theory, this is what has happened to the other six dimensions. It is theorised that they curled up just after the Big Bang. It is possible that if some variables in the Big Bang were different to what they turned out to be, some or all of these extra dimensions would have expanded. What would such a universe look like? Obviously, our 4-dimensional perceptions cannot imagine what it would be like, but the possibility of it happening remains.
String theory states that the electroweak and strong forces have the same strength at an energy of 1016GeV, and that gravity as well will have the same strength at 1019GeV. If all the forces have the same strength, an equation can be written to describe them. (At least, that's the theory.) At this stage, no-one knows what will happen if a particle accelerator could produce the required energies. We are certainly going to have to wait a while for the results, as the US Congress has blocked moves to build an accelerator powerful enough.
Originally there were five separate string theories, each one of them working in different situations. These, in 1994, were unified into a single "M-theory". M-theory may only holds true in eleven dimensions, which is different from the original ten.
The theory has not been fully fleshed out. There are many advancements still to make. It may be that and "F-Theory" (F for father) will emerge, a string theory that involves strings being present in either ten, eleven or twelve dimensions. We are still far from the utimate Theory of Everything.