In my view the Dark Ages happened because of the resentment, jealousy & anger Rome generated to its 'colonies' through 5 centuries of rule. Once the barbarians at the gates destroyed the Roman Empire in the west, anything Roman was reviled and obliterated - even the Latin language was largely abandoned due to its 'roman' connections so the systems of learning were lost too.
Don't forget, the Roman Empire in the East thrived long after Rome fell - it might be a more interesting "what if" to consider what effect on the world had the Eastern Roman Empire regained and re-took the west rather than abandoning them and consolidating in the east.
(I'm sure Brian will offer more input into this time period than I ever could....)
