Yes, demons and angels are in their own dimension and that isnt' a dimension we are supposed to try and mess with. Many do and find out the hard way that was a mistake.
And no, I do not believe in the so-called "gap theory" as I find that it does not stand up to what is taught in Genesis nor in looking at the totality of scripture. I believe in a literal interpretation of Genesis for very sound reasons. I believe God's Word for what it clearly says. Six days means six days - no more and no less. If God had meant something else He would have clearly said so. Please no Hebrew word lessons as I have done in depth studies of that as well.
Studied that one for over 20 years also and I find Dr. Morris' and Dr. Whitcomb's analysis to be in line with scriptures their as well....although I came to that conclusion through studying God's Word even before I was aware of those guys.
You are correct that God does not tell us everything (Deut. 29:29a) but there are many other things God does reveal to us (Deut. 29:29b) and the Bible is it's own best commentary; scripture interprets scripture. That is why I look at the totality of the scriptures related to a topic.
Funny, ..most people I've met who have studied some Hebrew enjoy Hebrew word lessons.