I imagined exactly the same. However, Planck experiments show that the universe is flat. It means, our theory was wrong - or the sphere is so big that we can't

I imagined exactly the same. However, Planck experiments show that the universe is flat. It means, our theory was wrong - or the sphere is so big that we can't notice its curvature, thinking we're living on a flat 4-dimentional surface (almost like with Earth in ancient times)