The term "The Dark Ages" was first used by Caesar Baronius. He was an Italian Cardinal and Catholic church historian.

It was in his twelve volume history "Annales Ecclesiastici" 1588-1607, that he used the term "The Dark Age" ("Saeculum Obscurum" in the Latin in which he was writing) to refer to the period between the end of the Carolingian Empire in 888 and the Gregorian Reforms in 1046.

Source: http://en.wikipedia.org/wiki/Caesar_Baronius

"Dark Ages" can mean two things. First, that the times referred to are not illuminated for us by written records. Second, that the times themselves were "dark" (violent, uneducated, impoverished) for the people living in them. Baronius probably meant both for the period which he labeled.

After Baronius, the term was used by others, first to refer to the entire Medieval Period, from the fall of the Western Roman Empire, to the rise of the Italian Renaissance (or roughly the 6th century to the 14th).

Later, historians objected that the High Middle Ages (era of cathedral building, troubadours, expanding cities, growing trade, expanding populations) were not so dark, and in the 20th century, the term began to be used mostly to describe the 6th through 10th centuries. (Back towards Baronius' original timeframe.)

Later still, historians began to object that these centuries were not so "dark" everywhere. The Eastern Roman Empire was still thriving for example, and the term has fallen into disfavor and become somewhat politically incorrect (among responders on this sub-reddit for example).

There are parts of Medieval Europe where it is still a very good descriptive term, however. One of these is British history (which, of course is one of the most important parts of Medieval history for English and American historians). British history from the 5th century through the 10th century was pretty "Dark" in both senses of the word.

We have very little information, and very few sources about what really happened in Britain (whether England, Scotland or Wales). In England, Roman civilization did seem to collapse. Cities were depopulated. Roman buildings decayed. Christianity was lost. Literacy decreased severely.

Quite a lot happened that we wish we knew more about. How did both Brythonic and the Pictish language disappear in Scotland to be replaced by Anglo Saxon derived Scots and Gaelic? How did the Angles and the Saxons come to dominate England so completely and rapidly? What caused the collapse of Romano/Celtic Britain?

It's all pretty dark. While I have some sympathy for Medievalists objections to the over use of the term "The Dark Ages", its a dramatic and descriptive term when applied to British history.

While there probably was some technological progress (in a very few areas) in Britain during this period, there was not much. In fact, there was a general technological backsliding, and technologies which had been used in Roman Britain (building in stone, making good roads, building cities) fell out of use.

When applied to Britain it is not a misconception that there was very little technological progress during the "Dark Ages".