There was another "Dark Ages" period in 1200BC [1] when dome building was again forgotten for 1400 years although it was quite usual in tomb building [2].
During that dark ages 500 year period most cities near the Mediterranean were destroyed, and it is attributed to the "Sea Peoples" [3]. Population in eastern mediterranean was reduced to 1/9 of its previous numbers and commerce that numbered thousands of ship transports ceased.
One can only imagine were we would be, if we had not forgotten printed type by 2500BC[4] or small gear mechanisms by 100 BC [5]
Non "Dark-ages" years are less than the "dark ages" years in recorded history, we just like to think that civilization improves over time.