there are something like 30 universal constants all with precise values. the masses of certain particles, the strength of certain forces, etc. we've measured these to a degree of precision.

there is no known reason as to WHY they are set at the values they are.

furthermore, it has been observed that, for a fair number of these constants, if the values were even slightly different from what they actually are, that the universe would be radically different. things like the inability of protons and neutrons to come together and form atoms, thus preventing any complex structure in the universe and life itself.

the fact that the constants are so finely tuned has prompted religious nuts to proclaim this as clear evidence that a god exists, because 'he designed the universe so perfectly fine tuned so as to support complex structure and life.'

if you are atheist, you reject this, and are left with the curious reality that our universe is indeed fine tuned to support complex structure and thus life. except you don't have an explanation, which is problematic.

the idea is, at the time of the big bang, a very large number of parallel universes all exploded from a single point, each of these universes having a different combination of values for the universal constants. much like the way we live on earth not because it was designed for us but because we wouldn't have evolved elsewhere, we exist in this universe not because it was designed for us but because the other universes were unsuitable to support life. in this multiverse, life happens somewhere, and it happens where it is most likely: this universe, this planet.

my theory extrapolates further on this: why only have multiple universes that merely tweak the values of fixed functions? why not have universes with a completely different set of functions entirely?

thus, any possible universe that can exist, does exist.

couple with the many worlds theory, and you have such an incredibly vast and most likely infinite number of universes, that everything you could conceivably imagine exists somewhere. that is inclusive of every work of fiction, every religion, every daydream and nightmare, everything. that is the nature of an infinite 'soup' of possibilities. something about an infinite number of monkeys and the works of shakespeare.

even if experimentation has shown that the monkeys mostly produce pages consisting entirely of the letter 's.' which ironically mirrors the theory, as most of the universes would likewise be rather uninteresting, consisting of a blob of energy which the rules of that universe prevent from collapsing into interesting or complex structure.