If you inflate a balloon, the surface area of the balloon increases and some dots drawn on the surface would get further apart but you couldnt say that the enlargement has a 'centre' The entire surface is simply expanding everywhere at once. The universe is like that with an extra dimension.
Still confused! With a balloon surface if you travel in one direction long enough you will end up back where you started. Does this analogy apply to the universe.
If there is no centre then either the universe is infinite, or it wraps around on itself like a balloon?
I believe whether the universe wraps around, or is infinite is still a matter of some debate. I know some researchers are currently trying to determine the size and shape of the universe by trying to see all the way to the other side and back to our side, for example.
In some cases with things in physics and cosmology, lets say dealing with 4 or 11 spatial dimensions you've just got to accept that our minds are not designed to be able to visualize some concepts, though we can understand and make progress by understanding the math.
Heres a random article introducing some ideas about the geometry and topology of the universe.
http://io9.com/5811706/if-you-keep-going-around-the-universe...
The Big Bang is better understood as a Big Stretch, which is still ongoing. As time progresses there just happens to be more space added between things. The balloon analogy breaks down because the galaxies themselves, or the dots on the balloon don't get bigger due to gravity. The expansion of space is only relevant on huuuge distances, not interstellar or even intergalactic, only when one gets to galaxy clusters and superclusters.
If it helps, think of the Universe as always having been infinite in size, even in the microseconds after the Big Bang. What was happening, is that the space between everything inside of the Universe, was inflating extremely rapidly.
