Board Thread:Questions and Answers/@comment-14.187.207.102-20200525150125/@comment-36009152-20200525234006

Hmm, quite possibility the dark substance could very well be Yu's consciousness that can become Yu, either in the past or be brought into life during the present time. Or perhaps he was physically sent forward in time. It's a good question, yet one that requires further chapters to help shed some light.

A hint is when Yu's memories were supposed to be the ones accessed, yet it was in fact Asuramaru's. So maybe Yu doesn't have any past memories to be unlocked because he's not the past Yu, maybe.