How do scientists know our universe is flat?
The observable universe is spherical.
The part of the universe we can prove exists is the part we can observe. As light travels at a set speed and in straight lines, we can only see so far from earth, as the light from some parts of the universe may not have reached us yet, as the universe is simply not old enough.
As the universe ages, that distance will increase, however we can only theorise as to what is beyond it. So even though scientists theorise it may be "flat", we cannot know it.