At a basic level, we kind of didn't have them. Despite what a lot of common knowledge (including this infamous chart) would have you believe, the world didn't just slip into barbarism when Rome fell. The idea is very eurocentric, ignoring the advances and civilisations that existed throughout the world. Mesoamerica, the Middle East and Eastern Asia all had complex, city building, stuff inventing cultures while the 'Dark Ages' took place in Europe.

Even in Europe, the analogy falls apart. Especially when you consider that the eastern HALF of the Roman Empire, what is now referred to as the Byzantine Empire, survived for the better part of a thousand years longer and maintained or improved on the technology that had been used in Rome.

This FAQ is a good resource and has links to tons of useful threads on the subject of the "Dark Ages".

"Dark Ages" are generally caused by the breakdown of centralized authority, and the social ills which generally attend it. What causes said breakdown of authority varies from situation to situation. Generally, what causes us to view an era as especially dark is the desire of later generations to justify the superiority they feel towards their predecessors. "The" Dark Ages, i.e. the era following the collapse of the Western Roman Empire, were caused by a whole host of things. I tend to think the most decisive being the loss of North Africa, first to the Vandals and then to the Muslims, and severing of other Mediterranean trade links which were vital to the economy of the Empire (others will likely disagree).

Beyond that, I think your question is somewhat malformed. Developing new technologies was never really a priority of the Romans or Greeks. It happened, sure, but the modern scientific/technological mindset would have been very alien to them. Thus, the question and the mindset laying behind it, "how much more advanced (technologically) would we be if the dark ages didn't happen?," condemns an era for not having the same priorities as the modern industrialized west. The loss of technology which did occur was largely a consequence of the loss of political and economic stability. It's hard to maintain industries when there are no more markets for your goods, and it's hard to maintain markets when everybody keeps stabbing each other.