occam's razor gunna cut ya!

Quite a few physicists reject the theories of multiple universes pushing their argument through by the use of Occam's Razor: we should not multiply entities unnecessarily, they say, and what is more multiplicatory than a possible infinity of other universes? They are quite wrong. Indeed, entities should not be multiplied, but the entities referred to in this principle are not physical objects, but rather, explanatory ones. The number of physical objects is irrelevant; consider the statement that Earth should be the only "Earthlike" planet in the universe, since postulating other worlds similar to ours would be complicating the situation unnecessarily. From what we know of planetary formation, there are likely to be many, many Earthlike planets out there, even though we haven't detected any yet. To proclaim that our planet is the only one of its kind in the Universe is, indeed, complicating the explanatory entity unnecessarily, because it would require a new physical law, something that would guarantee our uniqueness.

This same argument applies to the many universes. Multiple universes are not required but are certainly suggested by most cosmological theories today. We cannot detect them, of course, but only the most hardcore positivist would consider that to be a counterargument. On the other hand, postulating our universe as unique would require modification of said theories, an introduction of some kind of restrictive quality, a new physical law again. So the idea of multiple universes, originating, for example, through chaotic inflation, is simpler/more elegant than single-universe hypotheses.