There are two ways of viewing the concept of "His glory filling the earth."
One can see it as children do, perhaps, as air filling all space, or as something all pervasive, like space itself.
Or else, in penetrating into the philosophical nature of the problem, we touch on a certain Kabbalistic view by which the holy letters of the Torah are not the black designations of writing, but the white space surrounding them.
It is like the drawings of certain painters (or the designs of gestalt psychologists), in which image and background change places depending on the viewer's emphasis.
The world thus can be seen as an image against God Who is the background, or God can become the image against the background of the world.
In either case, God is not to be found in some other world; He is somehow intrinsic to this world, constituting its very existence.
Or possibly, the world manifests only the shadow of Divine existence.
Like a film projected on a screen: What we see are only the shadows cast by the light thrown against a moving series of negative pictures.
Thus, life as a whole can be justifiably called an illusion of passing shadows, which may hold us fascinated, but which has no more genuineness or reality than what we give them.
Because "only He exists," and the world is not another reality in addition to Him.
It is a shadow, or a small visible fragment of His infinity.
God is not elsewhere, in Heaven or in some invisible spiritual realm of being.
He is here and everywhere, filling all with that Divine essence which is being.
–Rabbi Adin Steinsaltz
From The Long Shorter Way, p. 221, by Rabbi Adin Steinsaltz