In the Miles Cosmology(1) there are many different universes floating around in a kind of meta-dimensional sea known as Ur-Space“. There are things living in Ur-Space as well, some of whom invaded our universe when it was young and were fought off by the Great Houses. This is known as the Yssgaroth “incident”. In regular parlance - it was the Vampire invasion that the Timelords repelled. See Crimes Against History:Ancient History.” Other universes are NOT parallel universes - nor are they Shadows as they defined in the Amber roleplaying game. Instead they are child universes that share common ancestry. Universes can give birth to other universes. The universes that are close to each other in Ur-Space are ones that tend to be incredibly similar in their development because they share the same Unigenes. Unigenes are similar to Biodata and DNA - a set of inherited traits that determine the development and/or timeline and fate of that universe. The presumption here is that while universes are not living beings as such, there will develop within a universe lifeforms capable of creating new universes. Universes further away in Ur-Space become progressively weird until you get to places where humanoid brains could not function. Universes do not effect each other. The Great Houses regard it as anathema to even think about attempting to contact the Great Houses (or equivalents) in other universes.
(1) - mostly taken from Lawrence Miles' essay in his book Dead Romance.