The idea of the Multiverse has its roots in the physics required to describe the Universe that we see and inhabit today. Everywhere we look in the sky, we see stars and galaxies, clustered together in a great cosmic web. But the farther away in space we look, the farther back in time we look as well. The more distant galaxies are younger, and hence less evolved. Their stars have fewer heavy elements in them, they appear smaller as fewer mergers have happened, there are more spirals and fewer ellipticals (which take time to form from mergers), and so on. If we go all the way to the limits of what we can see, we find the very earliest stars in the Universe, and then a region of darkness beyond that, where the only light is the leftover glow from the Big Bang.