I believe the original meaning of the term "Dark Ages" was in reference to the amount of written records available from the period. There are indeed very few records from about 500 to 1000. Therefore, that period is "dark" to historians. Doesn't necessarily mean that everybody was living a primitive lifestyle, they just weren't writing much down. :)
Let's first look at the onset of "the" Dark Ages in the sixth century AD. The Roman Empire was finished, nothing was happening in the sciences, and worse was happening in nature. The Italian historian Flavius Cassiodorus wrote about conditions that he experienced during the year AD 536 :
"The Sun...seems to have lost its wonted light, and appears of a bluish colour. We marvel to see no shadows of our bodies at noon, to feel the mighty vigour of the Sun's heat wasted into feebleness, and the phenomena which accompany an eclipse prolonged through almost a whole year. We have had a summer without heat. The crops have been chilled by north winds, [and] the rain is denied."
Other writers of the time described similar conditions : Procopius : "...during this year a most dread portent took place. For the Sun gave forth its light without brightness...and it seemed exceedingly like the Sun in eclipse, for the beams it shed were not clear."
Lydus : "The Sun became dim...for nearly the whole year...so that the fruits were killed at an unseasonable time."
Michael the Syrian : "The Sun became dark and its darkness lasted for eighteen months. Each day it shone for about four hours, and still this light was only a feeble shadow...the fruits did not ripen and the wine tasted like sour grapes."
Was this a local phenomenon? According to the book "Volcanoes of the World", Dr. Timothy Bratton has noted that there was a small eruption of the volcano Mt. Vesuvius in AD 536. Could this be the cause? It may well have contributed to the scene (although the eruption was much smaller than the big one of AD 79), but it can not really account for the similar conditions that were experienced around the world.
In China, "the stars were lost from view for three months". Records indicate that the light from the Sun dimmed, the expected rains did not eventuate, and snow was seen in the middle of summer. Famine was widespread, and in the midst of the turmoil, the Emperor abandoned the capital.
More here: The Dark Ages: Were They Darker Than We Imagined?
"the destruction and chaos accompanying the fate of the Roman empire [midway through the First Millennium] was all but total, the almost complete breakdown of the old order leading to a loss of the accumulated knowledge and wisdom of antiquity which was far from temporary."
LT, you may find the link in post #17 informative as the dates may relate to the Lost Tribes.