Ethan already explained this.

See "range of validity".

There are a few issues with the big bang model as presently understood--Ethan's written about them--but overall the theory works really well. Are the issues because we have something fundamentally wrong? Or because we are almost right but are missing a few details?

Well, the appropriate thing to do is use the model where it seems valid, and be more hesitant where it approaches the phenomena which aren't predicted all that well. And, as Ethan says, keep trying to overturn it by doing better.