well, that's not what the bible says... so...
even if you think it's a metaphor, I don't see any way genesis could possibly be interpreted as a metaphor for the big bang
Its not, but it was created around 2500 years ago.
Just like with the stories (if you're christain) Jesus told, they weren't true, they were dumbed down stories to represent something for people who had no literacy and barely knew much. The men who followed Jesus were mostly poor or middle class, they didn't go and learn like we do in first world countries.