Define nothing

Just suspend your actual concepts and start another paradigma.

Why is there something rather than nothing?

Well, why not? Why expect nothing rather than something? No experiment could support the hypothesis ‘There is nothing’ because any observation obviously implies the existence of an observer.
Source: http://plato.stanford.edu/entries/nothingness/

What is Nothing, anyway?

It’s not anything, and it’s not something, yet it isn’t the negation of something, either. Traditional logic is no help, since it merely regards all negation as derivative from something positive. So, Heidegger proposed, we must abandon logic in order to explore the character of Nothing as the background out of which everything emerges.

Source: http://www.philosophypages.com/hy/7b.htm#nothing/

What is no-thing?

Our first approach to this question already shows us something unusual about it. From the outset in asking this question we posit no-thing as something that “is” such and such, as be-ing. But plainly it has in fact been distinguished from just that. The question about no-thing—what and how it, no-thing, is—turns what is being questioned into its opposite. The question robs itself of its own object.
Accordingly, every answer to this question is impossible from the outset. For it necessarily starts out in the form: no-thing “is” this or that. Question and answer alike are themselves just as nonsensical with respect to no-thing.
(Martin Heidegger: What Is Metaphysics)

Source: http://wagner.wpengine.netdna-cdn.com/psychology/files/2013/01/Heidegger-What-Is-Metaphysics-Translation-GROTH.pdf – p.38