Oh cool, an Uber. I actually have
limitedknowledge of this tier.
But because of the lack of Ubers in POTW lately, I think another thing some people aren't considering with counters and checks is the fact that even though you can use any Pokemon in Ubers, the vast majority of Pokemon just aren't good enough to be worth a slot on your team. Even some of our favorite OU threats end up being there because of a niche (Mega Charizard X, Chansey even though it gets way too much attention), or just plain nonexistent (Talonflame, Greninja, Rotom-W)*. It's not as simple as saying something like Vital Spirit Primeape can beat Darkrai, because while it can, it's not a particularly good idea to use Primeape in Ubers. Same extends to Breloom, who still gets very little usage. And while Life Orb Technician Mach Punch does OHKO Darkrai, it can't switch in because Sludge Bomb OHKO's back.
*Going off of 1760 statistics
As far as prominent counters go, there's no way to fully counter Dark Void unless you're going for something less conventional. Sleep Clause and Lum Berry sweepers in general are things to keep in mind, though, since an opponent can and will take this to their advantage if possible. Outside of the concern with Dark Void, most of the earlier posts covered it. Xerneas and Arceus-Fairy are two Pokemon Darkrai wants to watch out for, along with [Mega] Blaziken, Scarf Genesect, Mega Mawile and [Mega] Scizor.