That question is Heidegger's famous question (or pseudo-question).
As Michael Inwood has written in his book Heidegger: A Very Short Introduction, depending on whom you ask,
"He [Heidegger] was (with the possible exception of Wittgenstein) the greatest philosopher of the twentieth century. He was (with the possible exception of Hegel) the greatest charlatan ever to claim the title of 'philosopher', a master of hollow verbiage masquerading as profundity."To ask Heidegger's question (a prejudicial question that, in itself, presupposes a reality of pure contingency) is to deny it. To ask "why . . . rather than . . ." is to ask for a causal agency, and such a causal agent is then itself "something." The question is meaningless and self-contradictory, pure and simple -- empty verbiage masquerading as profundity. The venerable maxim "Ex nihilo nihil fit"1 has no exceptions. Heidegger was a charlatan.
So then: The issue is not that there is "something"; the issue is the nature of that eternal and ultimate "something." Both sides in the debate have an ultimate. The ultimate "something" of atheism of the material monist variety is eternal impersonal matter 2. Material monism explains nothing. This worldview of a "godless material universe" is self contradictory (e.g., eternal matter is incoherent as discussed here), and further, it cannot account for immaterial abstract entities such as logic, absolute morality, minds, human free agency, or rationality. . .none of which can emerge from mere "matter in motion."
There are also the atheists of the pluralist sort, those who merely posit a plurality of eternal, incoherent co-ultimates. This incoherent reality is populated with such things as realms of abstract logic and abstract moral laws, chance, matter, minds, space-time, and a whole host of other brute facts. This is, essentially, atheistic Platonism. This incoherent philosophy explains nothing, as well as violates the principle of Ockham's razor, of which most atheists are very fond -- unless it applies to them. These Platonic brute-fact worlds of the pluralist interact magically via Platonic mysteries! These are even more brute facts. This is a great deal of question begging. We will deal with this version of atheism in a future essay.
All varieties of atheism contradict themselves and resort to question-begging presuppositions and special pleading. They end in futility and destroy the possibility of knowledge. So then, we know God exists because of the impossibility of the contrary (transcendental argument or apagogical argument via internal critique). Since atheism, on its own presuppositions and its epistemological "standard," is self-refuting, there is a God.
In Christian theism, ultimate reality is the personal Triune God who is the Creator and who sustains the universe. He is the source of all life, morality, rationality and the whole of creation. Again, it is ONLY Christian theism which is consistent and provides the foundations for all of human experience.