In a simple sense, “parallel universes” is a hypothetical set of multiple possible “universes” or “worlds” stemming from the non-deterministic nature of quantum mechanics (one of the most feared subject for physics majors). Once a feature in scientific literature, it has increasing been extrapolated as romantic notions in film and music.
In 那些年，我們一起追的女孩, Ko Teng and Chia-Yi were reminiscing about their missed romance when Ko Teng suggested that maybe in a parallel universe, they would have become a couple. That scene, I believe, must have left many viewers swooning.
Also in 周杰倫 - 你好吗 where the lyrics mourns of a lost love, the writer seeks solace in the possibility that in a parallel universe, they would still be together.
A scientist by education, it is interesting to see how academic concepts can be romanticized. Who says Science is boring?