Aggressive Expansion

You know how the Universe is apparently expanding but at a slower rate than it was in previous billions of years ago? Couldn't that just be because now there is more Universe to expand and perhaps at the point of creation where the big bang supposedly happened, it's still expanding fast, but then on the 'edges' it's just going slower because there's more universe now? Or is the theory that it's slowing down reliant on the fact that there was a big bang, therefore, a bang of extreme energy that would eventually run out of energy because it had expended it all through expansion? I don't know. It's really confusing and brainunwinding.