From what I have gathered, M-theory states that there are 11 dimensions. We live in 4 of these dimensions, and I believe the common understanding is that the 7 other dimensions are incredibly small. At the time of the Big Bang, these 3 dimensions expanded along with the universe while the others remained small. My question is this: Why did these other dimensions remain small? Also, if these dimensions had never expanded, does this mean that they would be found at the "center" of the universe?
No one knows IF the other dimensions really exist and no one knows why they remained curled up tightly. One hypothesis is that our universe would be unstable if some or all of the extra dimensions were large....planets could not form, elements would not be produced, and so we would not exist. Such extra small dimensions apparently weaken gravity so our universe can "exist"..larger rolled up dimensions might weaken gravity further..... Lisa Randall has done some theoretical work suggesting that branes make much larger dimensions than previously envisioned possible....or maybe it was the ADD model...maybe even a millimeter in size.....no evidence yet that I have read... We have a LOT more theoretical mathematical models than "real" worlds.....just because the math appears to work doesn't mean it has already done so....maybe tomorrow!! PS: There is no "center" of the universe.....as far as we know...