In theoretical physics, M-theory is an extension of string theory in which 11 dimensions are identified. Proponents believe that the 11-dimensional theory unites all five 10 dimensional string theories and supersedes them. Though a full description of the theory is not known, the low-entropy dynamics are known to be supergravity interacting with 2- and 5-dimensional membranes.
Kind of makes you wonder, what else can be out there.
And likely, the next thing you are wondering, is, how do theoretical physicists come up with this crap?