But a balloon does have a center even while it expands, and a grapefruit has a center.

In fact our observable universe of necessity has us at its center, since we haven't been able to look at it from any other perspective.

EDIT: In fact this Scientific American article seems to confirm that we are at the center of a "cosmic event horizen". Since there is no way to observe anything beyond this horizen, let alone travel there, we effectively are at the center of the visible universe.

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