Lack of technological progress in late Roman Empire

[floofthegoof]

One thing that seems to accompany the lack of technological innovation in the 'dark ages' is the issue of depopulation. The books I'm reading say that Europe in the dark ages were little single family farms interspersed in vast tracks of wilderness. What happened to the horde's of Gauls? Is there evidence of population decline in the old Roman cities as well? I wonder if there was some terrible plague that nobody lived to write about, because it really seems that a simple lack of central authority wouldn't cause what I think I'm seeing. Empty unused land in abundance. How can that be?