Originally Posted by Wikipedia
String theory showed how all the particles, and all of the forms of energy in the universe, could be constructed by hypothetical one-dimensional "strings", infinitesimal building-blocks that have only the dimension of length, but not height nor width. Further, string theory suggested that the universe is made up of multiple dimensions.
These "strings" vibrate in multiple dimensions, and depending on how they vibrate, they might be seen in three-dimensional space as matter, light, or gravity.
M-theory unified the 5 preliminary theories by asserting that strings are really 1-dimensional slices of a 2-dimensional membrane vibrating in 11-dimensional space...
Until some way is found to observe the yet hypothetical higher dimensions, which are needed for consistency reasons, M-theory has a very difficult time making predictions that can be tested in a laboratory. Technologically, it may never be possible for it to be experimentally confirmed.