M-theory
These six other directions can be explained if they are "curled up", so they are much too small to see. For example, by following the path of a spiral, it is possible to go a great distance along it without moving very far. The 6 other directions can be thought of as tiny spirals - strings can move along them a great distance but not seem to move. This can be looked at as a mathematical trick—a trick that has little to do with the real world that can  be seen and touched. Such tricks are allowed if they give a theory that can better tell us how things work.