The possibility exists that Equestria exists here on earth. Equestria is just on another plane of existence. You could be right next to your waifu and you wouldn't even know it.
I could be living inside Lyra's brain and she would never even know.
The multiverse is a stupid theory that falls apart every time you think about it for more than four seconds. Sorry, but your dreams don't exist anywhere but in your head.
Source: basic logic