To be pedantic, I said:To be pedantic, I will say it again. Unclear and unknown have two very different meanings and connotations. Saying it is unclear suggests (especially to someone new here) that there is information, of which there is not.
As for a third party possibly involved, I think that is very likely yes.
It's implied that their "history" includes the whole of it, and not just a part. We know they had contact of some form with the Alfar in the past. We know they closed off their realm millennia ago for reasons unspecified. We know there hasn't been open contact with the Alfar since then. At least, if we take Finn and Medb at their word, and trust they're knowledgeable about it.the history of the Dark Elves is unclear.
So we do, in fact, know some things about their history, even if the bulk of the last few millennia is unknown.