The theory there could be parallel universes is founded by the theory of the multiverse and that multiple realities are happening simultaneously with only one tiny difference to each one. This could be something as simple as someones haircolour being different or something much bigger such as Eistein not being born. Obviously this one tiny difference would then cause a chain reaction making a completely different reality than the one it was separated from when the difference in event occured. At this event, the timeline splits into two creating two separate realities - one with the event happening and one with it not happening.
With this theory you could also come to the conclusion that every little decision that happens could split into two or more different timeline realities therefore creating an infinite number of universes that is constantly multiplying.
Binary code credits to: vladart
Video by Christian Bodhi from Pixabay