I would rather ask what is nothingness?

It seems to me (both from reading this tread and generally thinking about the given topic) that there is a few possible outcomes.

1) Nothingness as a separate object among so called "somethings". For example: number 0 among other numbers or an empty world among possible worlds. In that case nothingness does exist. Although I would argue if that is really "pure nothing" as it possess the same "objectness" as something else. Number 0 has the same "numberness" as 1, 2, 123 etc.
2) Nothingness as a void in material world. In this case I think it depends whether we understand existence as a material condition. If X exists only if it has some material form, status, stance, whatever, then nothingness does not exist. Although the void still seems to be quite "full" when you think about the third case. As it can be understood as a prerequisite for "something", i.e. as an empty space in which we put material multidimensional objects.
3) Nothingness par excellence. Imagine that you have a bowl - this is your "time & space". You can fill it up with a soup or some snacks. Empty the bowl. What is nothingess? Is it the air now filling the bowl? Or maybe it is the empty bowl itself?

No. You throw away the bowl. Now there is nothingness.

And I don't think that the question is still relevant in the third case.

Let's leave God aside. I don't want this topic to become yet another religious war on God's existence nor I want to explain universe by some sort of deity. ;)

Shall we just use 'X' instead?

God is ineffable. — Tom Storm

"X is ineffable". How were you able to assign a predicate "ineffable" to X if you assume that X is ineffable?

Ok, we might say that if X is actually ineffable then by saying "X is ineffable" we are not talking about X, but, let's say, about a conception or an image (or our own fantasy if you wish) of X. Let's call that conception/image/fantasy/thought a fantasy-X. So we are saying something about fantasy-X and therefore fantasy-X is not ineffable. But if we are having in mind fantasy-X as we say X then to say "X is ineffable" is the same as to say that "fantasy-X is ineffable" which is not true.

But I must admit that I haven't properly separated predicates "incomprehensible" and "ineffable". Let's allow X to be comprehensible but ineffable. Then someone can actually think about X. We shall even allow to name that X. So someone can have X in mind and even say 'X' (as long as 'X' remains just a name and is not used as or accompanied by a predicate).
Even if this is the case, how can someone use predicate "ineffable" to an ineffable X? Once again, X is not ineffable if someone can at least say that "X is ineffable". So it seems to me that If X is truly ineffable then we even lack a proper predicate to name X's "ineffableness".

"Whereof one cannot speak, thereof one must be silent." (L. W., Tractatus §7). Or should we say in this case - "whereof one cannot speak, thereof one always remains silent".

God isn't a part of the known universe — Agent Smith

I agree with that. What bothers me is if God isn't a part of the known universe, how do I know that? For me to be able to say that "Michael is not in the kitchen" shouldn't I have some sort of notion what Michael is?

I guess our question requires us to provide a criteria of allowing something to be a part of the known universe. If I take perception and/or body senses as such criteria ("to be known is to be perceived") then how can I know that I never perceive God if I assume that I have never perceived him before and therefore don't know what God is. And if we take some sort of abstract thought as a criteria to be known, then how I was able to make such abstraction "God" if I can't know what "God" is?

In the first case it seems that it is said that God is not perceived just because we took such assumption (that God is something not perceived). And in the second case shouldn't we admit at the end that something is known what is not known?