Welcome!

I feel that child imaginary friends and hyperspace are very different phenomenons, but in any case they are both very interesting!

I understand what you mean of this 'missing' feeling.. It could be a mixture of things, though, like an existential hole that we always try to fill up, or also simply an inherent characteristic of being alive, because nothing is ever 'complete' but always changing..    

This also reminds me a bit one of the feelings I had a few times regarding dmt, that it seems somehow connected to the 'realm of the dead/souls', a sort of like primordial hyperspace soup of beings. So its as if it was the place where we came from and where we return to, and hence the sense of 'already knowing it'.   This is all of course more philosophical speculation but interesting to ponder about..   In any case gotta keep an open mind and not close to only one model or explanation. After all, thats the sense I have from dmt experience, that there is a Deep Mistery behind reality, beyond any explanation.