Because we do know something Craig.
While I do not know what is within the infinite microcosm, beyond the smallest thing I can imagine, I do know that there cannot be gaps, pockets, dots or even infinite minute areas of nothing, because there is existence; and there could not be existence, if any such nothing actually did exist. Also while I cannot fathom the infinte macrocosm, I know that it could not be otherwise, there could not be an infinite nothing within which everything else lingered. Now I have to apply this concept to every form of existence, not only matter but also energy and intelligence.
Also while it is not accurate to say that "the Infinite exists" nor that the "Infinite does not exist", we know of existence because we live within it. The Infinite is simply that, the Infinite!