a short sci fi story i read, by A C Clarke IIRC, started by talking about this flowing river of spacetime, with a vast amount of universes on its edges like eddies in an actual river. the short story was based on a universe where there was one star, one planet, and an interesting spacetime geometry
i think it's quite probable that we exist within a 'multiverse' other universes, with different constants/properties, different geometries, etc
why are we in this one? because this one is favorable to the development of beings like us. the anthropic principle, it's done as much as occam's razor when we need to defend against the relatively unevolved theists

(joke)
why are there universes at all? well, we dont even know why there is a background to THIS one (quantum vaacuum--its not really as vaacuumous as youd think) of spacetime, even in terms of general relativity, why would spacetime be a seemingly malleable 'structure'-like entity?
aka, what lies behind spacetime and the extensions within spacetime like electrons? what is behind the scenes, what is more primitive?
if there is no 'time' or 'space' beyond the big bang, i personally believe there still has to be physics beyond the big bang. the universe's mysteries can't just end at some fundamental particles, because we are left with the question of what makes THOSE particles tick? then we find out all over again, they arent really the fundamental entities
makes you wonder whether the search will EVER be over?