Space-time is the medium in which massive objects exist in our universe. That probably isn't a satisfactory description, but keep in mind that no definition will be as what I posted is all we really know.
Space does not have to expand into anything or displace anything. Objects in space that expand in a vacuum do not even have to displace anything, although they do expand into space. But keep in mind that you cannot conclude that a thing has all the characteristics of its constituents. Just because things within the universe have to expand into something does not mean that the universe itself has to expand into something. Metonymy works in poetry, not logic.
You're like an Okizaki fragment - you're lagging.
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