What they mean by "flat" is that it's generally isotropic, particular as you include larger dimensions for comparison. Only locally (that's a very relative term) does it "clump" into smaller things like galaxies and galactic clusters. There are a couple much larger gaps and clumps, but for the most part, it's flat.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."