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.

No, when Yuu dies than he comes back as a 5 Year old Kid with no Memories.

Whilst nothing is certain we could guess that past Yu's were clones that were unresponsive until having the dark substance implanted within them which gave Yu his personality/consciousness. The present day Yu is unlikely to come back as a five year old clone - there doesn't seem to be any more.