Apparently the confusion revolves around the meanings of the term, 'know' (I have knowledge that) and its
derivative term 'knowledge.' My use of either term will imply the other.
First let us be aware of some of the many different uses of the term 'know,'
I know I am a living person. (Innate, Intuition)
I know 2 plus 2 equals 4. (Prescriptive, rote learning or truth by definition)
I know how to tie my shoelace. (Being able to do something)
The encyclopedia is filled with knowledge. (Printed symbols of thoughts, ideas, and beliefs)
I know Lincoln was shot. (Documentary, Rote learning)
I know there is a God. (Blind faith conditioned by authority)
I know there are triangles. (Mathematical constructs)
I know (Fill in your friend's name.). (Recognition)
I know the sky is blue. (Conventional naming of sense experience that is common to most people)
I know there are other minds. (Inference from verbal, written, and/or general behavior of other life forms)
I know he is in pain. (Inference from behavior)
I know there are quarks, electrons, and atoms. (Indirect and constructed)
For most of us, no systematic method is used to support those claims of knowing with the possible exception of the the last one.
Besides being based on available evidence, minimally, and among other things, knowledge must be:
methodical (i.e., founded on a method of acquiring the evidence),
Some degree of PROBABILITY is a permeative character of all knowledge and evidence because, according to available evidence, no one can know absolutely all there is in the universe to be known past, present, and future.
All shareable knowledge is belief supported by shareable evidence.
All that we do learn, i.e., get to know, is founded on our perceptions of an assumed physical reality.
In the absence of these perceptions, there would be no conceptions of the world or things in it.
In any ultimate sense, we have only our perceptions and what we can conclude from them forming concepts.
It is important, however, to distinguish those ideas that have a basis for deduction and "induction" and those that don't.
Those DISTINCTIONS are "perceived" by us also.
Even the premises of "induction" are presumed descriptions of an assumed reality "beyond" the (our) mind(s).
Bearing in mind all of the above, why, then, do we claim to know there is a universe?
It would be the height of irrationality to think otherwise.
If we thought otherwise, we would be faced with the problem of explaining the source of all our experiences, for instance:
The historical persistence of events.
The recurrence of some perceptions and not of others.
The fulfillment of some predictions and not of others.
The verification of some claims and not of others.
The coherence of some claims and not of others.
The commonality of some experiences (such as solidity, liquidity, etc.), and not of others.
We know that claims of verifiable existents, i.e., common perceptions of them, are true while claims to unverifiable existents are epistemic nonsense.
We cannot rationally deny that something exists, whatever the nature of that existence.
We may differ, argue, about what to call that something, but "it" does exist in some sense of that term.
It doesn't matter that the perceptions, predictions, claims, etc., are themselves not physical.
What matters is that they were somehow "caused" to exist -- even if they are only dreams.
However, we, also, have thoughts that are not immediately, at any rate, sense related, i.e. deemed to be caused by some external, to our minds, i.e., brains, event.
We know there is a universe, whatever its nature.
And finally, if the questioner did not know, i.e., believe on the basis of evidence, that there is a universe, SOMETHING existing, how COULD he, WHY, and to WHOM or WHAT did he raise the question?