The many worlds theory suggests that there are an infinite number of universes that exist outside of our own. This theory was developed by physicists studying quantum particles. In quantum physics some things can have properties of either particles or waves and there is no way to determine which one. This influenced the many world theory because in quantum mechanics things have a wave-particle duel nature, so each fate of a particle is carried out, but just in different universes.
According to this theory you are not the only you. You are living in an infinite amount of universes, each with different possibilities of you. All the possible alternatives that ever existed in history exist in a different world. Anything that could have possibly occurred in our past actually did happen in a parallel universe.
Many scientist refer to these universes as giant bubbles floating around in space as shown below.