Universe, is a rather overwhelming concept. We have always studied the Universe as everything there is. There is nothing outside the Universe or rather, there is no outside to the Universe. Universe is all there is and everything that exists is in this one particular Universe. And then, we study the origin of this universe, its history, its expanse and even its future, but what if there are other Universes? And we, being minuscule parts of one of those Universes cannot even think of existence of others? But, the Scientists have thought and thus, brainstormed a rather fantasy-like Multiverse theory.
The Name gives away everything in this case. The multiple Universes that co-exist with our own universe. Since 1930s there have been plenty of attempts in explaining why this is a possibility. One of the theories suggests that the Universe made up of particles, can only have a finite number of arrangements of those particles. Even though the number is very very huge, it is not infinite. And as the Mathematics gives, there can be 10^10^122 possible arrangements. So, after these many arrangements, there must be repetitions thus giving rise to similar other universes. There can certainly be other universes which differ an arrangement of just one particle or just two particles.This can explain the possibility of Multiverse existence.