Eternal Inflation

Eternal inflation is the most mainstream multiverse theory. During cosmic inflation, the early universe expanded at incredible speed. In some models, inflation doesn't stop everywhere at once. Different regions stop at different times, each forming a separate "bubble universe" with its own Big Bang.

This links to the anthropic argument: if physical constants varied even slightly — the strength of gravity, the mass of the electron — stars and life could not exist. Maybe every possible combination exists somewhere, and we simply live in one that allows us to be here.