The real problem is, the "Dark Ages", as idiot americans call it, are actually better known than most people realise. However, the notion was actually a later nationalistic one brought about with every european nation basically saying, "oh, we are the true successors to Rome," and other such nonsense.
Really, though, the "Dark Ages" was more about the collapse of the Roman roads and economic structures. Rome controlled a lot of land, and trade within the empire was a lot easier than it would have been otherwise, currency being a major factor. Once Rome "collapsed" (it just became the church and started taxing its successor states in other ways), the roads, more or less, became far more dangerous. This segmented the various nation states, and it helped prop up the feudal system.
Since no big nation existed to police the roads and areas, it came up to local provinces to police themselves.
And, as history has shown time and time again, turmoil is mostly "brief" after a great empire falls, only for the people who once formed it to gather together and work through their problems.
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Also, the loss of petrochemicals now would be, more or less, a blip in the fire; and the fact is, we'd never truly run out of oil.
However, it'd be rather expensive.
But diesel is going nowhere as you can make oil to run in diesel engines from literally *anything* biological. Garbage, food waste, lumber....
Ethanol too, although it is far less efficient to do so.
Also, if the US ever gets off its ass and allows the production of Thorium reactors, the lack of an energy source will never practically occur.
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Finally, the closest equivalent in modern times to the "Dark Ages" would be the segmentation or loss of the internet.
That is all.