My idea of it being an illusion comes from strange personal experiences. I'll try to not sound like a nut job while I describe this.
I've travelled extensively for someone as poor as I am. I've been to almost every state in the contenental U.S. and overseas several times. In between the U.S. & Europe as well as in between the east and west coast, I always felt like I left one reality and entered another. Especially through flying to and from Europe so many times. It felt like something I can't describe. But it was if I left one dimension and entered another.
Without trying to sound crazy, you should be open minded to a very slight possibility that astronauts might feel they have gone from one dimension into another, provided that they weren't government-produced machines trained to not think for themselves.