What's your narrative about how the existence of the universe came about? How can something just randomly appear out of nowhere? I guess the universe has to have already existed forever

I think it's perfectly reasonable to say, "I don't know." Ascribing supernatural significance or explanations to what was likely a natural phenomenon isn't helpful.

Time just exists within our universe, so there is no "before the universe existed" or anything like that.

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.

Let's discuss it over a cup of tea. Can you fetch my teapot? It's in a solar orbit between Earth and Mars and so small that it's impossible to see from Earth even with telescopes, but it's totally there (trust me bro). If you can't find it, you're just not looking in the right place.

Many people, much more intelligent than you or I, have philosophised about the lack of human understanding in particular areas. What you are doing is attributing the birth of the universe to the "god of the gaps". Your only evidence to support your conclusion is a lack of evidence to the contrary, and that is not a strong foundation to any argument. Any assertion that is made without evidence should be dismissed without evidence (see Hitchens' Razor). I could just as easily claim that the entire universe and all living things came to existence last Thursday, and all evidence to the contrary came to exist that way as false evidence (see omphalos hypothesis, last thursdayism).

My take on it is that there was simply too much nothing so it exploded.

To be honest, I don't feel any obligation to explain the origins of the universe to myself or anyone else. It has nothing to do with my atheism at all. Maybe it was the big bang? Preceded by a big crunch? Honestly hard if not impossible for anyone to know for sure.

Religious folk can't explain it either, to be fair. "God did it" is a pretty weak story. What, he waved his magic wand? What was there before? Who created god?

All these questions are unanswerable by nature, and therefore irrelevant (even if it's fun to speculate!).

Mine comes from my own terrible interpretation of wave existence/ string theory. I don't currently think things where this dramatic but it's still fun to consider.

There was a dimension, one far far smaller then our own. It is energy, hotter then any quazar. It's not at all stable.

The little universe became too small for the energy it contained and exploded. It ripped a hole into (one or many) neighboring dimensions. Like drops of blood in gigantic pools of water, the engery is dissipating into the void.

Now if you must belive in a "god". Think of how we are beings with millions of tiny lifeforms living on us. Not just our cells but an entire ecosystem of bacteria, virus, parasites, and other micro beasties we try not to think about. Those have life forms living on and in them. If this dimension of energy was like a living being like us, perhaps the proto life forms that built us where some of the things living on it. A tiny virus that was living on a cell of foot fungus living under "god's" toe nail before it exploded into our dimension and eventually became us.

Tbh, it really could just have randomly appeared out of nowhere. Could you accept that if it were proven?

I tend to take a route of absurdism through life, so I would frankly be delighted if that were the case.

https://youtube.com/watch?v=BWv02kYyvo4

The best answer is “We don’t know”.

The superior answer is “Through Jesus all things are possible, so write that down…”