Flat universe concept is confusing! Universe only appears flat bcz the distances that we probe are way too small to detect any possible curvature in the universe.Ex: for we humans on earth or for an Ant on a balloon earth or balloon always appears FLAT. .Only observable universe(OU) is Flat.Every observation we can ever make is confined to OU.

This is explained in Universe versus the observable universe.

While special relativity constrains objects in the Universe from moving faster than the speed of light with respect to each other, there is no such constraint when space itself is expanding. This means that the size of the observable universe could be smaller than the entire universe; there are some parts of the Universe which might never be close enough for the light to overcome the speed of the expansion of space, in order to be observed on Earth. Some parts of the Universe which are currently observable may later be unobservable due to ongoing expansion.

Some parts of the Universe may simply be too far away for the light from there to have reached Earth, but despite the expansion of space, at a later time could be observed.

Both popular and professional research articles in cosmology often use the term "Universe" to mean "observable universe".

This can be justified on the grounds that we can never know anything by direct experimentation about any part of the Universe that is causally disconnected from us, although many credible theories require a total Universe much larger than the observable universe

. No evidence exists to suggest that the boundary of the observable universe corresponds precisely to the physical boundary of the universe (if such a boundary exists); this is exceedingly unlikely in that it would imply that Earth is exactly at the center of the Universe, in violation of the cosmological principle.

It is likely that the galaxies within our visible universe represent only a minuscule fraction of the galaxies in the Universe. According to the theory of cosmic inflation and its founder, Alan Guth, the entire Universe could be at least 10^23 to 10^26 times as large as the observable universe.