In perturbative string theory, spacetime is a manifold M mathematically defined like
M = N x R^4
where N is a Calabi-Yau manifold.
In M-theory (non perturbative regime), spacetime is a manifold M mathematically defined like
M = Nī x R^4
where Nī is a G2 manifold. There is really few known about these manifolds even in mathematics (compared with famous CY). You can see that there is much more information about CY.
For some elementary information can see
http://www.explore-math.com/mathemat..._manifold.html http://encycl.opentopia.com/term/G2_manifold
I don't know much about these but I still can sure you that they will do not work (M-theorists think that topology of universe may be explained by a G2)
I don't know the details of G2 manifolds but i know that are again an archaic choosing for explaining universe because also are usual differentiable manifolds like famous CY and therefore cannot explain spacetime foam.
Again string M-theory is a disaster like an irrelevant attempt to theory of everything.