The phrase is a bit of a Renaissance invention. They defined themselves in opposition to what they considered a backward period in the history of the continent. Usually, though, the phrase refers more specifically to the period before 1000, and even then, as post #2 points out, it is a gross oversimplification. As far as the High Middle Ages go, so many important aspects of western civilization were beginning to emerge (law, philosophy, universities, vernacular literature) that it's especially hard to see them as "Dark Ages."
Modern historians do not believe that it is really appropriate to characterize the Middle Ages in Europe as a "Dark Age." They say that the term might apply at some times and places, but that it is excessive to claim that it applies to all of Europe for the whole of the Middle Ages.
For example, even in the early Middle Ages, there were monasteries all over Europe. These monasteries served as sources of education and as places that could and did keep scholarship alive during the period. In the High Middle Ages, universities were starting to spring up in places like Bologna (1158) and Paris (1200). These can be seen as evidence that the Middle Ages were not as "dark" as they have been made out to be.
dark ages were dark because there were many dangerous things going on like the:
politic and religious problems