Markus Schwarz wrote:the universe is not expanding into anything. While the expansion is often described by an inflating balloon in popular science books, this analogy fails when one asks your question. On cosmological scales the distance between two objects, galaxy clusters say, is indeed increasing with time. It can be described solely in terms of "intrinsic" quantities that don't require any extra dimension. This can well described mathematically. Unfortunately, it is not very intuitive and the balloon analogy is the closest I know of.
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