Well, in String Theory in Theoretical Physics the question is really more about fundamental constants and why, given the infinite possibilities, a universe exists with constants that are set "just so" to give us all the macroscopic stuff like inverse square laws which makes this universe so hospitable to human life. From the persepctive of probability and within the shortcoming of String Theory, there's no reason that the incredibly improbable event that is existence ought to be, at all.
In String Theory, the collection of all possible universes is known as the Landscape, and some physicists have proposed the Anthropic Principle - that if the fundamentals weren't the way they are we wouldn't be here to figure them out - as the explanation surrounding this universe vis a vis the Landscape.
Leonard Susskind has a book about this: "http://www.amazon.com/gp/product/0316155799/ref=wl_it_dp/102-6838729-1449754?ie=UTF8&coliid=I16YIL3S0BVAQT&colid=4AYE7AC6JTRZ"
ST, while the primary focus of most of the world's theoretical physicists, hasn't given up much in terms of experimental predictions or testability. Yet the problem of the incompatibility between General Relativity and Quantum Mechanics (and the Standard Model) remains a great focus in the field. Loop Quantum Gravity is a runner-up theory to String.
A recent book for lay people by Lisa Randall is a good introduction to String Theory: "http://www.amazon.com/gp/product/0060531088/ref=pd_bxgy_b_img_b/102-6838729-1449754?ie=UTF8"
Matt
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