I'm new here. I suppose you're assuming the universe didn't always exist, so my comments may not be of interest. The universe has always existed. Since you've introduced God into the question, I think you're right to find it problematic about a starting point for the creation, and the problem doesn't go away and gets even more troublesome if one sees that time itself may only have appeared with creation, which is why we say God is atemporal.Why didn't the universe always exist? Because since God is supposed to be outside of time and is supposed to have always existed, then how could God have used a point in time to start creation? Any thoughts on this?
Since God has entered the question, I came across this which made me think: that to imagine creation starting in some way is to imagine a God without creation somehow let's say 'before' it happened. And so we have a creator without a creation, and so what does God even mean like the sun without its characteristic light. Is the Creator the Creator without its creation?
This destroys other characteristic qualities. If God were without its creation, then we're saying a change happened in God when it created, in fact when it became Creator. So now God is no longer changeless or eternally the same but subject to change like created things. You can see all sorts of problems from violating the essential names or qualities of God, but this post is probably too long now, cheers.