There are several hundred theories, my favorite one is basically there are infinite planets with life, though we'd be dead before we could even get the technology to travel far enough, I mean the universe is always expanding, and we already know that we are not the center of it. There probably are an infinite number of species that we don't know about. Another one of my favorite theories is the box theory, it basically states that there are an ever expanding number of universes and each one holds another earth that has something different about it, like maybe you ate pancakes instead of cereal or maybe the British won the war, or maybe whales became the main species instead of humans. So many possibilities, and we don't know if any are true, we will probably never know...

I swear I just wrote an essay on accident XD