here is columbia physicist brian greene:
In an infinite universe [more precisely, I would say, in a sufficiently large universe], most regions lie beyond our ability to see, even using the most powerful telescopes possible. Although light travels enormously quickly, if an object is sufficiently distant, then the light it emits - even light that may have been emitted shortly after the big bang - will simply not have sufficient time to reach us. . . . The important point is that regions beyond a certain distance...lie beyond our cosmic horizon. . . .
Using a two-dimensional analogy, we can compare the expanse of space, at a given moment of time, to a giant patchwork quilt (with circular patches in which each patch represents a single cosmic horizon. Someone located in the center of a patch can have interacted with anything that lies in the same patch, but has had no contact with anything lying in a different patch, because they're too far away.... The same idea applies in three dimensions, where the cosmic horizons - the patches in the cosmic quilt - are spherical, and the same conclusion holds: sufficiently distant patches lie beyond one another's spheres of influence and so are independent realms. (The Hidden Reality, 32)
and here is poe, from eureka (1848).
We comprehend, then, the insulation of our Universe. We perceive the isolation of that - of all that which we grasp with the senses. We know that there exists one cluster of clusters - a collection around which, on all sides, extend the immeasurable wildernesses of a Space to all human perception untenanted. [This a spherical space, for Poe.] But because on the confines of this Universe of Stars we are compelled to pause, through want of further evidence from the senses, is it right to conclude that, in fact, there is no material point beyond that which have thus been permitted to attain? Have we, or have we not, an analogical right to this inference that this perceptible Universe - that this cluster of clusters - is but one of a series of clusters of clusters, the rest of which are invisible through distance - through the diffusion of their light being so excessive, ere it reaches us, as not to produce on our retinae a light impression - or from there being no such emanation as light at all - or, lastly, from the interval being so vast, that the electric tidings of their presence in Space, have not yet - through the lapsing myriads of years - been enabled to traverse that interval. . . [We have a right to infer] - let us say, rather, to imagine - an interminable succession of the "clusters of clusters" or of "Universes" more or less similar. (Library of America Poetry and Tales, 1328-29)