As a hardcore materialist, the realization I am almost certainly wrong about the existence of physical matter is always difficult to get my head around.

My brain is wired such that intuitively, no matter how wrong it is, when I dream of flying through some mountains and then reflect on WHERE those mountains were/are, I cannot escape the false sense that the mountains are "in my brain". Even though no such mountain will be found by dissecting my brain.

When I do grasp that there is only now, only here, and only something reminiscent of mental activity, the insight is brief before I fall back into neurons and matter and such... This is probably an evolved survival mechanism to view everything this way... But I do think when I have momentary clarity that there is a greater understanding of what reality actually is, which does have scientific implications regarding things like "true" AI which are hot topics in the modern day, as well as how something could "come from nothing", or how something could exist before the big bang/time. Basically the "big questions".

I saw Stephen Hawking discuss "vertical time", and it is almost impossible to understand such a thing directly without experiencing what eternity and timelessness is.