In a 2011 interview, Columbia University physicist Brian Greene — of the book "The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos," — explained that we're not really sure how big the universe is. It could be a really, really huge but finite. Or else, as you travel away from Earth in any direction, space could just stretch on forever, which is probably the way that most of us imagine it.
But if space is infinite, that means it must be a multiverse with infinite parallel realities, according to Greene. Here's the reasoning: Think of the universe, and all of the matter in it, as the equivalent of a deck of cards. Just as there are only 52 cards in a deck, there are only so many different forms of matter. If you shuffle that deck enough times, eventually, the order of the cards you deal must repeat itself. Similarly, in an infinite universe, matter eventually would have to repeat itself, and arrange itself in similar patterns. A multiverse, with an endless number of parallel realms containing similar but slightly different versions of everything, provides a neat, easy way to accommodate the need for repetition [source: Greene].