From what I understand, no one is sure why matter is not spread evenly in the universe or why it seems the universe is expanding faster than expected. Is it possible that there was more than one Big Bang or a series of little secondary Big Bangs causing matter to be spread haphazardly and giving space a boost in speed to counter gravity?
On large scales, matter actually seems like it is spread uniformly throughout space. On small scales, of course, it isn't - but that is due to the effect of gravity. If you imagine an a universe of infinite size where the density of matter is the same everywhere, all you need is a tiny fluctuation in density at a particular point and more matter will then be attracted to that point, so you can eventually go on to form a galaxy or cluster of galaxies around it.
We think there was only one Big Bang (in our observable universe, at least) taking place everywhere simultaneously because the evidence we see is that everything in the universe (on large distance scales) is moving away from everything else. If there were multiple Big Bangs, then presumably you would see some faraway galaxies moving towards each other, or at least a more complicated relationship between the distance of objects we see and the speeds at which they appear to be moving away from us.
This page was last updated June 27, 2015.