Our ancestors had an "explanation" of how flint works, which enabled them to work it into useful tools, which in turn enhanced their ability to survive. They didn't need an "explanation" of where flint comes from.
The OP erroneously views the universe as a container that can be empty. The universe is the things that exist, not a set that may or may not contain things.
Or... the universe includes both the container and the contents. The container exists, even if there is nothing "in" it.
Can you have space with nothing in it? I think you can.
As I understand it, the universe is the space; the "things" in it are just "wrinkles" in the space. So yes, I would say you could have a universe with no things - perfectly flat.
If a singularity were the 1st thing ever to exist....
A singularity is not a thing. It is "where" (though it is not a place) you can no longer ask, "Where did it come from?" No logic is possible beyond that "point".
So I kind of agree with you: The universe can not be explained in the same way that a tree can be explained - but neither can you "prove" that it can not be explained. At some "point" (which is not a place) all logic goes offline; the screen goes blank.