Not what you would think.
, as in "Space, the final frontier...," the absolute space that Newton posited, is a word first used in this way by Milton:
Space may produce new Worlds
says Satan, imagining Earth, our world, as such a new world in space
. (It's worth recalling that Galileo, whom Milton visited, is the only contemporary person named in Paradise Lost
: other than he, Charlemagne and Columbus are the most recent figures mentioned.)
Milton's slightly older contemporary Descartes was thinking about space at the same time: measuring it with a coordinate system, and declaring that the concept itself was incoherent. There had to be ether everywhere to make distance possible. If space is empty, it's nothing, and if it's nothing, there's nothing to measure, and nothing there. If the sun is an AU away, it's because an AU of ether separates us from the sun, and we can measure the depth or length of that quantity of ether. (I think a similar argument about the strange emptiness of space is part of the inflationary theory of the early universe, but that's just me being wooly-headed, probably.)
Anyhow, it may be that Milton also couldn't quite countenance empty space, because he has God say, six books later:
Boundless the Deep, because I am who fill
Infinitude, nor vacuous the space.
So Milton fills space as well, with God.
Or does he? God does "my self retire" to give way to freedom. If that retirement is absolute, as Empson suggests it might be, i.e. that God might "abdicate," then we're in Gnostic territory, and space becomes a true abyss. So it may be that Milton's thinking anticipates Einstein's even as it parallels Descartes' and Newton's.