So I googled it. The answer is somewhat confusing to me, although I accept that there is probably no theory of the universe that has a simple explanation. Anyway, an excerpt of the heuristic explanation from Quora:
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Imagine a point particle: its supposed to be zero-dimensional. Lets represent it as a dot on a piece of paper. I realise that if you look with a magnifying glass, it does have dimensions but lets not! Now then, experimental physics has shown us that a point particle doesnt live in one place, or any place for that matter, but rather inhabits all possible places until its actually observed. To be able to do that, a point particle has to have some other properties.
First, lets extend the point particle upwards, off the surface of the piece of paper. It now has one dimension. Lets not make it too long, it can be the Planck Length upwards from the paper. Thatll do for now. (The creation of other point particles will extend the dimension for us).
Next thing is it can start to wiggle a bit. Its not stuck on the paper any more. Then, lets smear it sideways - it now becomes a two-dimensional sheet. Its become a brane. Lets now smear the brane in a direction perpendicular to the way its standing - it becomes three-dimensional, i.e. a three-brane. Lets now move the three brane away from the point that everything started. Lets call that direction time.
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Finally, in answer to the question, what caused the vibrations to start in the first place? Probably a random fluctuation in hyperspace which caused what we know as the Big Bang, although its also likely that it was the collision of two branes in hyperspace, due to gravitational oscillatory motion. It could also be due to a catastrophic phase transition in a previous Universe. It might have been started by someone in a lab in another universe. Possibly, a previous universe stretched flat and forgot how big it was, in which case it became small again and started to expand. Or, the observable universe became a bubble universe which expanded so far that the observable universe became a bubble universe etc etc
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I'm not sure that any non-physicist can intelligently decide between the various theories of physics.