Space time in the presence of masses is curved. But during the time of Big Bang it's presumed that all the matter in this universe was at a single point, so it must have been super dense and had very high mass. So space time at that point would have curved very deep down but today it's almost flat. How can it be so flat today? Or is it due to our observation limit (like from surface our Earth seems flat, but in reality when observations are taken from far away we can see the curvature)?
$\begingroup$ But during the time of Big Bang it's presumed that all the matter in this universe was at a single point This is not true. In a cosmology where the universe is spatially infinite, it's spatially infinite at all times. $\endgroup$– user4552Nov 7, 2018 at 14:43
We don't known if it was ever a point, General relativity tells you that it must have been very very small, approaching a point, but if you take the theory too far in this direction it breaks down, and we say we have a singularity, one of the goals of physics is to produce a theory to cure this singularity. Then you ask why spacetime is flat now, it isn't. Spacetime isn't flat. Space alone is, at cosmological scales, flat. That depends on the density of matter and energy present in the universe. It appears that our universe has the exact density to be flat.