If you are a materialistic secular atheist, then by definition, you believe that everything, that is all matter, energy, space, time and physical laws are contained within the universe.
Therefore, the cause of the universes existence must also be included somewhere in that list.
So, based on the latest from science and your own experiences and opinions, given that you preclude any outside agency, that is, God, as a cause, are you happy to shrug off the question of origin, and simply say, "don't know," or do you have a sensible theory you would be willing to defend?
Well I acquainted myself with de sitter universe over ten years ago when Waldo McFroog was debating with me but I confess that although I could explain it then, I have forgotten it all now. Too tired to look i up at the moment. A final state as I remember.
A void is what's in a shoe box when the shoes are taken out. Not very scientific but true, and incapable of creating anything except a bed for the cat maybe.