What do you mean you don't remember what Dark Monkey Matter is?!?
It was first mentioned in MS1 #3 and been referred to a few times throughout the story. Most notably in MS1 Annual #1, in the story "Dark Monkey Matters" which I'm not gonna link you to it because, frankly, I'm in a bit of a hurry. Click a cover image on the right & get to reading!
A Future Family Reunion!