A 10 dimensional universe

Superstring theory calculates that the universe has ten dimensions. Very early on there was an amicable split. Four dimensions expanded and went on to produce the observable universe's space and time (3+1 dimensions). The other six shrivelled up so as to be invisible, although they still exist. As Dean Overman remarks in 'A case against accident and self-organization':

For the purpose of the formation of life, this split was fortunate, because carbon-based life could not exist in any other than three spatial dimensions. Gravity would not allow for stable planetary systems unless it functioned in three spatial dimensions because it follows an inverse square law which requires the force of gravity to decrease as distance increases. In four spatial dimensions, the force of gravity would fall to a fraction of one-eighth its power (rather than one quarter) for every doubling of distance, and in five spatial dimensions, the force would fall to one-sixteenth its strength for every doubling of distance. Moreover, in more than three spatial dimensions, the force of electromagnetism would not function in a manner which would allow for life, because electrons would either spiral away from or into the nuclei.