I think it is 2D because the universe is not that thick?
I will add that it is said the universe is flat, but that does not mean 2d flat. Flat is a mathematical construct, which means parallel lines stay parallel and triangle angles sum to 180 degrees. So it is 3d flat at least as far as we can observe.
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I will add that it is said the universe is flat, but that does not mean 2d flat. Flat is a mathematical construct, which means parallel lines stay parallel and triangle angles sum to 180 degrees. So it is 3d flat at least as far as we can observe.