Big Bang Problem

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and then, y did it start spinning?

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and y did it explode?

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First, there is absolutely no part of the big bang model that says *anything* about spinning. Not sure where you got that idea. Maybe a reference?Why space expands is not exactly known. It is something we observe that it *does*, and we can determine *how*, but we can't say *why* it does it like this and not differently.

There are several ways to test it, yes. Experiments and observations has been done that shows that what the big bang model says about the universe is correct.Some examples is the discovery of the background microwave radiation that was predicted by the model and later observered. Other things includes how the development of stars and such should look like when we observe 'deep' into the universe.

In short: Because the big bang theory has made predictions which has later turned out to be correct.It is based upon indirect evidence in that no person was around at the time (obviously) but that is not a requirement.