I believe the frontrooms, backrooms, and all the other _rooms are a part of one large universe. The frontrooms are our entire observable universe. Things just clumped together in perfect order. The farther away a _room is, the lower the chance matter has of going there
Each is in a pair. the midrooms are a sort of transport point of matter, and they each exchange matter with each other. They are more likely to trade matter with a pair over another random object.