Why did Europe go into the dark ages after several years of incredible advancement and prosperity? We've gotten used to the idea of steady gains and prosperity

Why did Europe go into the dark ages after several years of incredible advancement and prosperity? We've gotten used to the idea of steady gains and prosperity, but there are cycles that include book burning, knowledge destruction, and civilizational declines. Ancient Aliens forces the viewer to ask what civilizations were even doing with odd constructions and what purpose they served; the only difference is they suggest that extra-terrestrials were involved. Extra terrestrial involvement may seem outlandish to some, but the question of why any civilization was doing what it was remains.