There seems to be a common theme among the complaints to nerf this card, that being the player claims someone used 6 black holes on them one after the other, in the very early game. ANY card is going to be powerful if used 6 times within the first few turns. 6 rush creatures, 6 good CC cards, 6 good permanents; 6 of anything in the early game is powerful.
I can understand people getting fed up with dark matter, but in that case, nerf the god, not the card.