The whole "Dark Ages" thing is a little contoversial, because it depends on your point of view. If you were living around the Mediterranean basin, things weren't so bad. If you were living in the periphery of the empire, civilization basically disappeared and reverted to tribal living or feudalism.
One of the big changes with Rome was that at some point, they became too big. Earlier in Roman history, the Romans would either kill everyone (Ceasar's conquest of Gaul killed a substantial proportion of the population) or begin a process of Roman-ifying the populace. The Roman armies kept the peace and spread the civilization.
The problem is that you had this built-in tension between keeping Rome (the City) under control, which was extremely challenging, and keeping the provinces in line and secure. Emperors had to spend fortunes subsidizing the City with grain, festivals/circuses and other support. Getting those fortunes meant levying onerous taxes, and collecting in the provinces meant enpowering the local Roman officials and military -- which in turn created rivals to the emperor.
Another issue is that as time went on, gaps in agricultural technology fundamentally changed the society. The core of the Roman republic was citizen solders and civil servants -- many of which were yeoman farmers. As crop yields declined and demand grew, these farmers went out of business and consolidated into much larger estates that were dependent on slavery.
All of these things resulted in changes to the character of the Roman system. Roman armies were not made up of Romans. Roman generals were not necessarily loyal to Rome. It just turned into a big mess.