Why not cut out the middle man and just say the universe is "self-existing"? Why do you need to have an explanation for how the universe came about but feel totally satisfied with the rationale that god always existed? (hint: it's because you've been brainwashed since birth to think that way)
Why can't the universe, the one we know, be self-existing?
Because there was a beginning of the universe, the one we know, and we know it has a beginning.
Why then can't things which have a beginning not be self-existent?
Because there was a time or a point between non-existence and existence when things having a beginning belong to the non-existence camp, they were still nothing but at most possibilities.
Then some entity that is existing prior to these non-existing things except as possibilities turns them from nothing but possibilities into really existing things and not only possibilities, and this is the beginning of their real existence, for which they owe it to the prior to them existing entity.
And the ultimate prior existing entity that starts the the chain of prior existing entities bringing about the beginning of posteriorly existing things, the ultimate last most anterior and end prior existing entity is the one and only self-existing entity which is God.
That is why I say that God belongs to that category of being which is self-existent, and God is the only member of this category.
As a concrete example, every man was nothing but a possibility until a man and a woman prior to him conceive him and the woman bears him in her womb so that at full term she gives birth to him.
The man from his mother and father has a beginning and therefore it cannot be a self-existing entity.
Now everything in the universe we know has a beginning for owing their existence to some prior agents, which prior agents have a beginning and thus are also not self-existing beings.
Until the chain of prior entities giving rise to posterior entities, reach the ultimate end anterior entity which is self-existent and is the only one agent that is self-existent, that agent is God.
Susmariosep