why would we have a clear understanding of the cosmos if the the big bang is thought to be fact? All it means is we have an idea of how it possibly started. That's it. It doesn't suggest that, aha! we now understand everything about dark matter, zero point science, quantum physics and the like.
Again, the idea the universe is expanding is just that, an idea. There's just as much chance that there is an end to it or it's not expanding at all. Why? Because in all honesty nobody really knows what the dickens is going on.