Problems understanding naturalistically the beginning of the universe

[Wryetui]

Hello to everyone (I have seen a post that is like this one but the forum adviced me to create another thread since that one was too old).

This matter "worried" me most when I was an atheist. I deeply believe that God created ex-nihilo the universe and I find it correct due to a number of reasons, but in this thread I am interested in speaking only naturalistically, in order to consider if actually the atheist cosmology is plausible now that I have a better understanding of the world and to eventually find a solution to this struggle that my atheist friends now (and myself many years ago) have. According to the Wikipedia: "The Big Bang is a scientific theory about how the universe started, and then made the groups of stars (called galaxies) we see today. The universe began as very hot, small, and dense, with no stars, atoms, form, or structure (called a "singularity"). Then about 14 billion years ago, space expanded very quickly (thus the name "Big Bang"), resulting in the formation of atoms, which eventually led to the creation of stars and galaxies. The universe is still expanding today, but getting colder as well.", and this is what I believed. But my main question is, since: "The universe began as very hot, small, and dense, with no stars, atoms, form, or structure (called a "singularity"). Then about 14 billion years ago, space expanded very quickly", since the universe actually began, where did it begin from? The universe is all we know for sure (and sometimes not even for sure), but how can something exist outside the universe? Where was this very hot, small and dense structure since the universe "didn't happen" yet?

Thank you, and I remind that I would want this thread to remain naturalistic, without interference from any kind of religion account.