Another problem with string theory, as far as I understand it (and it is not far) is that it aspires to be a theory of everything. Is that not to be aiming to high? I mean, when or how can we with any confidence say that it is nothing more to know about the universe.
The question you pose here is a perfectly valid one. Generally speaking, the more we learn, the more new doors are opened to new unknowns. In other words, the more we know the more we know that we don't know. As knowledge is a never-ending quest, it would seem like a Theory of Everything is a contradiction or oxymoron. In the sense that you have questioned it, you are right.
But that isn't what is meant by the phrase "theory of everything'. I will explain...
As far as we know right now, there are four fundamental forces which describe the interactions of all matter. They are the Weak nuclear force, the Strong nuclear force, the Electro-magnetic force and the gravitational force. There were, originally, four separate theories for describing each of these four different forces.
During the later part of the 20th century something called The Standard Model was developed, in which the weak, strong and electromagnetic forces were all combined into one unified theory. Essentially, it was shown that the weak, strong and electromagnetic forces were each different manifestations of the same thing and the Standard Model showed how they were related and why they each have the properties that they do.
All except gravity. We have not yet been able to include gravity into the model and put it on the same theoretical foundation as all of the other forces. We consider the combining of the three forces to be a Unified Field Theory. If we could also include gravity into the theory it would be a Grand Unified Field Theory or A Theory of Everything.
Theory of Everything simply means that all of the forces of Nature (weak, strong, electromagnetic and gravitational) can be described by one unified theory. From this theory, all of our familiar equations, such as Newton's Laws, Maxwell's Equations, Quantum Mechanics, General Relativity would all be derivable and related to each other through this one Grand Theory.
It doesn't mean that we know everything. It just means that all of the know forces have a common foundation and we know what they are.
Now I'll get a little weird on you...
In principle, if we achieved a Grand Unification Theory, we could then build a gigantic computer (Deep Thought anyone?) and program it with the fundamental equations of the universe and then press 'start'. As that program cranks away, a universe, not unlike our own would gradually evolve inside the computer.
It is fun to think of our universe this way. It is a giant simulation inside of God's computer (which he programmed with the same equations.) Occasionally he jacks-in and tinkers with it or even walks among us. Sometimes he sends his graduate students (angels) in for other tasks. This model also makes it easy to imagine different realms of existence. For instance, how it is possible for God to be outside of the universe, yet everywhere in the universe at the same time.
Think about it. It's fun to speculate.
). It is just that I do not know what holds the three forces together and exclude gravity. I do not have the faintest idea if there is something in the theoretical framework that forbiddes there being a fifth force, not yet discovered. And I do not know if this fifth force could very well exist even though it is not depicted by the Standard Model or String Theory. As you can see, there is a lot I do not understand.