Well, the universe can also be said to have always existed whereas 'always' means for all points in time that make sense. So 'before the big bang' makes as much sense as 'north of the north pole' (thanks to Physicist for this analogy).
You are also mistaken to think that the big bang is some kind of explosion which of course it is not. An explosion takes place within space but the big bang is the expansion of space itself.
Of course I'm only a layman in this regard but if you don't believe me we have a physicist here (just lurking behind the corner) who you can ask and who is much better at explaining these things than me.