What you are asking for is a “first cause”, or “a reason why everything exists, at all”, rather than simply not existing. This question is often presented by religious people in order to make the cosmological argument, which would say: “the Universe couldn’t come out of nowhere, there had to be an original cause for it to exist”, which then they claim it is their god.

However, the problem is trying to adjudge the first cause to something that exists, has existed or used to exist other than the Universe itself simply pushes the question forwards, because for that very thing anyone else could ask: “And what created the first cause?”, therefore “So the Universe exists because God created it. So what created God?”. When it comes to religious explanations, this problem only grows, because you’re pushing the first cause from something we know it exists (the Universe itself) to something we do not have proof of its existence, which ironically makes it even a weaker explanation.

In the end, there is probably a physical reason, but we just don’t know about it.