What Alric said . . . you're missing huge amounts of information.
"Dark Ages" is old, bad terminology for the first part the early Medieval period, which is roughly 5th century to 11th century, mainly in Europe. There is an incredible amount of diversity in religion, society and material culture across that window, from Moorish Spain to the later Roman empire to the Lituanians who remained pagan until the late 14th century, to the Norse, to the Danes, to the Merovingians to the Saxons to the Irish.
My rant is partially informed by "Dark Ages" basically meaning "We're 19th century historians who have no idea what the hell we're talking about and we don't like anything between Rome and the Renaissance so we're going to ignore it. Also, we're British historians, so we're going to ignore the Irish, because they're dirty savages, and the Mohammedans, because they're dirty savages, and pretty much everyone else, because racism is still in vogue and so forth."
Additionally "Dark Ages" pretty much always seems to mean Northern Europe. Generally speaking, folks who use the term "Dark Ages" are thinking of Europe, as the rest of the world was certainly not dark and going through all sorts of neat cultural and technical developments at the time. Hell, the "Dark Ages" encompasses the whole migration period in Europe and most of the Viking culture from, what, 7-800 on through the 12th century?
Additionally Tolkien, The Lord of the Rings, the whole Middle Earth Drama, is pretty heavily based on various old English and old Norse sources, the ring of the Nibelung, that sort of thing. The only time non-Northern-European cultures show up is the the Haradrim and the Easterlings.
Eitherway, my basic thesis is that a return to "Dark Ages" is a return to magic, which time period saw the life of most of the worlds great Magicians, Merlin, and so forth, and great Magic Swords, from Excalibur to Durandel and Cortana to Zulfiqar to Hrunting to Gram, and a good number of others. The Dark Ages were also a heydey for Dragon slaying, with the Tarresque, Nidhogr, St. George's various enemies, all packed in around the edges.
Likewise, the Lord of the Rings is stuffed to the gills with Magical deeds and occurrences. The Fellowship had two human members out of nine, the rest being non-humans, all of whom had some degree of magical or greater than human ability, if only counting the hobbits ability to go unnoticed at critical moments.
So... What realism are you looking for, exactly?