Well, logical tells us that everything must start from something, so what was the conditions for matter and dark matter to be created. I honestly don't think it'll ever be answered, but at the time, flat earth theory was logical in the knowledge they had available.
But how did the Sumerians...
Where the **** did all the atoms, gases and etc etc come from?!
Well, I believe the Big Bang was the consequence of a singularity of a black hole, and also our universe was born from the destruction of the previous.