Maybe you can look at SN 43.12-44 where the Buddha listed 33 ways to describe Nibbāna such as: nibbāna, unconditioned, uninclined, undefiled, the truth, the constant, the deathless, the freedom, the haven, etc.
For a noble discipline, nibbāna is known but can only be tasted at the arahant level. You can look at this SN 12.68 with the simile of water in a well for your reference.
So, I have given you references to show you that “nibbāna can be known”, do you have reference that says “nibbāna can NOT be known”?
Your definition was “The Unconditioned is but an expression for absence of the asavas .” which is a bit unclear with that word “but” which means “only”, “nothing else except”.
That’s why I gave a counter-example of inanimate object which is clearly “absence of the asavas” but is nevertheless conditioned.
You were asking “What is everything?”, you didn’t ask “What is all?”. In my 2 questions, I put “Everything is not self”, I didn’t put “All is not self”. As I said, I aware of SN 35.23, so I didn’t want to confuse others by saying “All is not self” or making a new definition of “the all”.
So, because you asked for definition of “everything”, I gave you definition of “everything” as
“all that are either conditioned or unconditioned”
For reference, you can have a look at AN 10.58 about “everything” (sabbe dhammā). The Buddha said in that sutta “nibbānapariyosānā sabbe dhammā” which were translated as something like “everything reaches nibbāna as perfection”.
Are you ok with that or do you want to use your own definition of “everything”? Please let me know.