Quantum mechanics is widely recognized as one of the most successful theories of all time, confirmed again and again by observation and experiment. The Many Worlds Interpretation of Quantum Mechanics, as proposed by physicist Hugh Everett III, tells us that it's not correct, though, for us to say that the quantum wave function is "collapsed" through observation. Rather we are only "observing", and the Many Worlds - the other parts of the wave function that we are not observing continue to exist. I agree with Everett's proposal that these other parallel universe outcomes are equally "real", but I understand why people find this so boggling: "how can there be more than one version of me?" is a question I hear regularly.
There was a paper published by physicist Jean-Marc Lévy-Leblond analyzing the Many Worlds Interpretation which Hugh Everett particularly liked, and it said this:
"To me, the deep meaning of Everett's idea is not the existence of many worlds, but, on the contrary, the existence of a single quantum one."
This is why I propose that the fourth dimension is all you need if you believe there is only one possible past and one possible future for the universe: it's like our frog is looking down a single street that stretches towards infinity, both in front of him and behind him. But since the quantum world is probabilistic, we know that there is more than one "next possible outcome" from any given instant, which means our frog always has many possible roads branching off, any one of which he could find himself turning on to: and those branches occur within a space which is orthogonal to spacetime, the fifth dimension. But our frog still can't see the whole picture. In the same way, you and I can imagine how Everett's Many Worlds can easily include versions of the universe where the Boston Marathon bomb attacks didn't occur in 2013, but we know that there is no chance, zero probability, of us observing that version of the universe now.
Enjoy the journey!