you are in a dream within a dream...its like a taco bell..which is inside of a mall...which is inside a city...in a state..that's in a dream..that you are having...which is inside another dream...which...IS..IN..SIDE..YOUR...MIND...
backa chicka lacka dacka aba gaga nana (dramatic music continues)
If I stab you in the face, you die. If I were imagined, that would not happen.
At any rate, your brain couldn't possibly process all the neccessary information required to construct this universe from imagination. It would require the brain to continually be generating new content while simultaenously retaining all prior content.