what does ‘the universe is flat’ mean? How can it be flat when there are stars, planets, galaxies etc. everywhere we look?
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It means the universe doesn’t curve, so parallel lines remain parallel.
It’s a flat, three dimensional infinity.
The alternative is that it does have a fourth-dimensional curvature, and parallel lines either intersect or veer away from one another. If that is the case, it may form a finite hypersphere rather than being infinite.
The observable universe is a different matter entirely, that’s a three dimensional sphere simply due to the finite speed of light.
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