Hugh Everett III suggested parallel universes as one of the possibilities in 1954, and they've been extremely popular ever since. All of this is just a theory as far as science goes, but there are people going public every day claiming that they found slips between dimensions and managed to go to different worlds. These are some of their stories.
The Mystery of Ong's Hat
Ong was a man who claimed his hat disappeared in another dimension after he threw it in the air. There's a town in New Jersey by the name Ong's Hat where the story originates. It's a ghost town now. In Ong's Hat, the Institute of Chaos Studies was founded by three scientists. Legend says that they mastered interdimensional travel using the chaos theory and that they currently reside in a dimension that never developed human life, but is full of plants.