I somehow feel aliens and demons and everything else if they exist are from here, not outer space, but here alongside us, very much a party of humanity. No matter what era we are in they are there but if we are woodsman they appear as forest beings, in tune with nature, fairy, but if we are religions then they appear as saints or Mary, they are always better than us at something, they have more skills in the forest, more knowledge about God, and today they are better than us, they have craft that can fly faster and go farther, of course now that we have craft they got upgraded.
So very much this parallel world has always been here. Some cross over, others come back, few choose to live in-between.
Didn't the book A Wrinkle in Time use the concept of folding space? And somewhere in the fuzzy back of my brain I think another sci fi book used the same concept.