To answer this question we first have to inquire  into what “Nothing” is.  Heidegger believed it to be something. For example, he saw the desire for Nothing as different from having no desires. But, this treatment of Nothing as something leads to a logical absurdity.

Premise 1: Nothing is better than great sex.
Premise 2:  Pizza is better than Nothing.
Conclusion: Pizza is better than great sex.

Nothing is not prior to the Not. Rather, the Not is prior to Nothing. The Not reverses the truth value of statements from true to false and vice versa like a switch. Heidegger wrongly treated Nothing like a substance or a name just because it was a noun in language.

Therefore, Nothing cannot be a state of affairs unless it is compared to something (i.e. Not something). So, logically something has always existed which renders this question meaningless.

This leads us to the next question which is why is this “something” the way it is? Why is the universe ordered in this way? For the answer to this question I turn to the anthropic principle. The anthropic principle basically states that we see the universe the way it is because we exist.

The reason why the Big Bang occurred about 10 billion years ago is because the universe needed to be old enough so that some stars will have completed their evolution to produce the elements out of which living things are made, and young enough so that some stars would still be able to provide energy to sustain life. If this was not the case we would not be here to ask the question.

For better explanations you should turn to the work of cosmologists and physicists who are tirelessly working towards a better understanding of the universe.

Related posts:

1. Everything from Nothing?
2. Why is There Something Rather Than Nothing (Part 2)
3. A Bad Explanation is Not Better Than No Explanation
4. A Short Refutation of the Kalam Argument
5. Big Bust Cosmology