I take this proposition to be almost axiomatic, a truism, a necessary truth. The domain of everything consists of all concievable things; even things such as the set of inconceivables (where inconcievables are undefined); all objects, propositions, truthmakers, logical atoms and operators, and the space in which any logical system may subsume.
We may distinguish between ‘everything’ and any such thing that may be other than such a term ‘everything’ could capture. If the latter could be an object of genuine distinction; without some absurdity (such as: ‘everything’ captures those objects and referents that may be impossible either logically or conceivable, or absurd in some other way), we may introduce the notion of a Noumenon; but this is not necessarily needed for such a constructible set ‘everything’ to maintain, even eliciting the objects within such a set are not necessary, merely that it is there would suffice.
Kant maintained that the experience and knowledge could only be grasped by means of an active part of human nature (or conscious nature) to operate in such a way where all things are forced under the principle of seeking categories; what these categories may be are not terribly important (for this post) but may be debated. It would seem to me hard to argue for something so basic and primal as the phenomena that is immediately present to us. When we feel an immediate pang of pain, or that capture of laughter; we hardly come to the first thought or experience of questioning whether we have such an experience at all, or whether there is such a thing as a quale; but we come to reside in that experience. Humean nature is also true in that there is a brute fact-ness of how human nature is enslaved by experience.
It is a misnomer to deride the notion of categories; to deride any given notion of categories would be fine, but categories as a wholesale notion would be harder, insofar as any correlation system may be made. Association is but inherent to conscious experience. Whether such an experience is veritable, is immaterial to that brute insight.
It is however, presumptuous to pose that human experience is objective experience. If we are to speak of objective knowledge (in a putative sense of the term); we may note it as being in some way indexical to the principles or conditions of experience itself. Talk of ‘subjective’ and ‘objective’ can be muddied in that respect.