how can scientists say the universe is flat, when it seems to go in all directions?
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In curved space, Euclidean geometry doesn’t apply. Two parallel lines don’t remain parallel. If you travel far enough you may come back to where you started. If you take three 90 degree turns to the right, you may not make a rectangle.
In flat space, Euclidean geometry works. Parallel lines never meet. You can travel forever without coming back to where you started. And if you make three 90 degree turns to the right, you will form a rectangle.
Flat space is not two-dimensional, though. That may be the source of your confusion. Flat space is just Euclidean space — except where an object like a star or black hole creates a slight curve. But those are just small bumps in space. Even the most massive objects in space are small compared to the universe.
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