The Dark Ages weren't "dark" in the sense that many understand them, but they were a period of fragmented power, small empires, and warring dynasties - much as much of the Roman period was, but the fact that their structure disintegrated is undeniable.
In their case, it all tied back to lines of communication and an increasingly inefficient taxation system, which lead to declining revenues for Rome, rot at the fringes, and eventual atomisation.
The 'dark ages' witnessed the high mark of Western Christendom, which had its own fascinating form of 'international' unity and an impressive legacy of achievements, and it certainly shaped modern Europe. I mean, I think everyone knows that 'Dark ages' is a made up label from centuries ago during the Enlightenment era.
Either way, neither of those labels are endorsed by any serious modern historian as far as I know.
And quite a number of low marks in terms of industrial production, technical knowledge, public health, collective wealth, individual freedoms and so on. In fact, without the rising Arab civilization undertaking huge efforts to acquire and preserve what the Western countries were discarding as "unchristian", we would have lost even more knowledge than we did.
There are so many reasons the "dark" attribute stuck and survived for almost 700 years (it wasn't an Enlightenment construct but rather a Renaissance one, although technically coined even earlier). It might be too generic a term to survive the contemporary level of analytical debate of history, but ignoring it would be a disservice to the cause of science and progress.
What makes you claim this? The growth of population, economy and individual freedoms continued through the Middle Ages. Ancient Rome was not a particularly "civilized" place if you look at it by, say, medieval English standards.
And lots of technical innovations were made during Middle Ages (both through interaction with Middle East and Asia, and through independent development). Technical knowledge accumulated. Public health learned some lessons - yes, some of those very slowly, it took 200 years to fully recover form the Black Death - but the population and economy grew, except for this plague anomaly which the ancient civilizations would hardly have handled any better.
Throughout Middle Ages, agricultural output grew and new tools were brought in and new crops were introduced, as well as crop rotation in farming (from just farming to fallowing and then to three-field rotation).
New scripts were developed, e.g. Carolingian minuscule enabled faster writing and producing of books (prior to the printing press which is seen as the starting point of Renaissance but was developed by medieval people).
Regarding individual freedoms: slavery, common in ancient civilizations, was largely abolished in Europe during medieval times. Magna Carta is clearly a medieval document.
Medieval times were not dark. Development was not as fast as later on, but it existed, almost continuously, with some (mostly local) setbacks.
One of the odd things about the fascination with Greece and Rome as the basis of all civilization (ahem) is the failure to understand how alien those civilizations were.
Not to discount the advancements of Arab world during this time period, but this is not an either/or thing and is much, much more complex than you are portraying it. Large amounts of knowledge were actually preserved by the church (mostly in monasteries) and the parallel church structure set up during the latter periods of the Western Roman empire helped cushion the decline of the West as Rome shifted East.
The "Dark Ages" were not dark (which is why historians have almost universally quit using that term in favor of "Middle Ages") and are a very complex and interesting time period.
I highly recommend Dan Carlin's excellent Hardcore History episode "Thor's Angels" . He spends four hours talking about just this very specific period (what happened in Europe when Rome declined), and spends lots of time talking about the church's role in stabilizing what could have been a whole lot worse.
Actually, the idea comes from Petrarch, who was a late Mediaeval writer who was upset at the disrespect paid to ancient Latin and Greek literature and culture during the Migration and Mediaeval periods. Of course, Renaissance, Reformation, and Enlightenment intellectuals all picked up on the idea for their own reasons.
It is a historic fact that Constantine the Great moved the capital of the Roman Empire in 330 to Constantinople, where it remained the center of the Empire for the next 1100 years.
To be more precise in Nova Roma or Constantinople (modern day Istanbul) and this since the 11th May 330.
It remained the center of the Roman empire until 1453, this was long after the Roman culture in the west was almost completely destroyed. (and with it western Christianity turned into a tool of oppression)
So while the West during these times faced oppression and feudalism, the Roman empire certainly did not.