To have humans exist and not have religion is impossible. There will always be that one guy that looks to the sky and asks "Why?". And to be fair, we haven't made it very far in that school of thought.
Also, we weren't exactly on the fast track to begin with. Or at least the world collectively wasn't. There were certain civilizations that sprung up (Greece, Rome, etc.) that were definitely doing things right, but no other civilization could keep them from falling.
And while these empires were on their way down, the empires in the east were actually in their own golden age until a certain series of events dragged them down eventually.
The "Dark Ages" really only happened in our eyes because there wasn't much about it written as there is for the ages after it. It was only the invention of the printing press that propelled us forward.
Also I think you should read [url=http://www.cracked.com/article_20186_6-ridiculous-myths-about-middle-ages-everyone-believes.html]this[/url]. It really provides clarification as to what the Dark Ages really were, and debunks popular myths commonly associated with it. Yes, it is a comedy site, but everything is supported.