eli5: How do we know the universe is flat?

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ive read that the universe is flat, as opposed to a saddle or a U shape but when I look into how they’re measuring it, I just cant understand.

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They look at the cosmic microwave background very carefully. That is the glow from the big bang at the last moment when the fireball had expanded enough to cool and become transparent. If space had a positively curved structure (shaped like a ball) the microwave background would look one way. If it had a negatively curved structure (shaped like a saddle) it would look a different way. The actual cosmic microwave background looks like neither of them! This was a huge shock as a flat (meaning Euclidean geometry like) space seemed really weird and unlikely. And it is really weird and unlikely, but it’s what some very sensitive measurements show.

To piggyback on OP’s question: I just had a realization that my previous thoughts about the Big Bang have been flawed. I can wrap my mind around an initial “explosion”, but I visualized it exploding in an expanding pattern in all directions simultaneously. That included both up and down, as well as 360°.

Honest question asked from a place of ignorance: why was this so?