User blog comment:LuffyPirateKing/Luffy and his belly/@comment-1595565-20121021173205/@comment-3213092-20121022164016

He killed Thatch with a knife to get the Yami Yami..

It's clear that the Yami Yami allowed him to steal Whitebeard's Devil Fruit but we don't know what happened under that dark blanket and how he stored it inside him.