I fthere was no before, then there couldn't have been an after. and we are quite obviously after, according to your theory. =)
Actually, there is no 'after' in a sense either.
There is no after the universe any more than there is a before the universe as described by the big bang. The big bang merely describes the universe as-is. It can't explain what happens 'after' the universe, or 'before' the universe and it can't necessarily explain if it makes any sense to consider before and after anyway.
All it can say is that the smaller the time dimension the hotter and denser the universe was, the greater the time dimension the cooler and more spread out the universe is.
To us, time is divided into Past, present, future. When Time is 0s there is no past, so trying to work out what happened before it, according to our models of time, is practically impossible.
What happens before time starts? What happens after time ends?
How can something happen if there is no time for it to happen in?
Big questions.
There is an 'after' Time=0
There is a 'before' Time=maximum
All we can describe is the universe between these two points (if the latter point exists at all)
so big bang/evolutionary thinkers don't have a theory for where the universe came from?
There are some interesting ideas, but no solid theories. As I said, it might not be possible to use the laws of nature to deduce how the laws of nature came into being...though we might uncover some of the meta-physics, its unlikely we'll know the whole caboodle.
However, many people that accept General Relativity/Big Bang/Evolution have their own ideas about how the universe got here. Cavediver is the biggest big bang/relativity guy in these forums right now, and he believes that the Christian God brought the universe into existence and our theories merely describe that which God created. Read some of his posts to get a more accurate idea on his thoughts.