Strip everything away in your mind — the stars, the vacuum, space, time, the laws themselves — and try to reach the thought of sheer nothing. It is oddly difficult; the mind keeps smuggling something back in. And the moment you succeed, the question detonates: why is that not how things are? Nothing would have been so much simpler.
Leibniz, who first pressed the question in this form, thought the answer had to lie in a principle of sufficient reason: for everything that is the case, there is a reason why it is so rather than otherwise. Grant that principle and the chain of explanations cannot simply hang in the air. Each thing in the universe is explained by another, but the whole collection still needs a reason — and that reason, Leibniz argued, must be something that exists necessarily, by its own nature, rather than something that merely happens to be. For him that was God; for others it is an abstract necessary ground. Either way, the regress has to end in something that could not have failed to exist.
The brute-fact replyMaybe there is no why.
Not everyone accepts the principle that launched the argument. Perhaps, the brute-fact theorist says, the universe simply exists, full stop, with no deeper reason — and the insistence that there must be one is a human craving reality never promised to satisfy. Every explanation has to bottom out somewhere; the only question is where. The theist stops at a necessary being; the brute-fact theorist stops one step earlier, at the universe itself. Neither can prove the other has stopped in the wrong place.
What physics can and cannot doThe limit of the ask.
It is tempting to hand the question to cosmology, and cosmology has extraordinary things to say about how our universe unfolded from its earliest moments. But notice the gap: a physical account always begins with something — a field, a vacuum state, a set of laws — and explains how more something followed. That is a spectacular answer to a different question. The philosophical "nothing" is the absence of the laws and the vacuum too, and it is not clear any empirical theory can reach behind its own starting conditions. This may be the one question where the honest answer is that we do not know, and perhaps cannot — which is itself worth sitting with.