As to how parallel universes were born, the scientists had put forward a lot of hypotheses and conjectures. The most popular one was the 'Observer Hypotheses', which suggested that the observation of the observers caused the collapse of the quantum state and the birth of parallel universes.
The simplest example was the thought experiment named 'Schrödinger's Cat'. According to the 'Observer's Hypotheses', before the box was opened, the universe was in the quantum state where two probabilities overlapped. When the box was opened and the 'Observer' saw the result inside the box, the universe would split into two. The two universes were almost identical, except that the cat was alive in one universe and dead in the other.
