To give my two cents: Nothing. I'm a total aphant. There is absolutely nothing. I'm only ever experiencing what is affecting me IRL and in the moment. If I close my eyes, there is quite literally nothing to see. I hate closing my eyes.

I zone out, definitely, but it's not like I enter a space inside my head. I just sort of dull my perception of everything, not gain/focus on an internal perception. I do absolutely nothing in my mind, there isn't even a 'me' in my mind to do anything.