Where do atheists think the universe came from at the very start

I don't really have an interesting in justifying my beliefs or a lack thereof, but I'll humour you to satisfy your curiousity.

I don't know what occurred prior to the Planck Epoch of the Big Bang. Singularity conditions mess with physics something fierce, making for a basically opaque window. There are some interesting hypotheses which are plausible within existing data, but not presently verifiable. Heck. Before might not even be the right word, given the hypothetical effects of a singularity upon space-time. There's a distinct possibility that those conditions existed for an infinitely long time.

But it's really a great big question mark. If you want to pretend that your deity lives in that question mark... hey, whatever floats your boat, so long as you aren't also pretending your vision of this deity is an excuse to do harm.

I'll make no assumptions, personally. Not that there was a before. Not that there was a creation. Not that there was a creative force. Not that the force if it did exist was sentient, let alone omniscient, nor that it wanted to be worshipped, nor views us as its special people, nor is even still there. I especially won't assume that it conveniently matches with the deity described in your mythology, which proved to be basically incorrect about the formation of the Earth, unless you want to squint really hard and call it an allegory. Hell, if there is something genuinely 'god-like' out there, a quick glance about the universe is enough to suggest that it's a lot more complicated than our primitive tales could conceive.

Easy...Go check out Large Hadron Collider, since you have obviously have an open mind and have studied your Bible/Koran/Talmud extensively, you shouldn't have too much difficultly in continuing this trend and do a bit of study of what their doing at the LHC. They are actually being able to prove the existence of dimensions beyond the four we perceive. While you're at it, check out String and Membrane Theory (M-Theory).