Flat-earthers, don't feel vindicated. The flatness problem of the universe, whose nature is beyond the anti-intellectuals' grasp, is a cosmological puzzle that extrapolated from the Big Bang model.
First raised by astrophysicist Robert H Dicke, the flatness problem deals with the relative density of matter and energy in the universe (Ω). Whether the density is less than, equal to or greater than 1 (or so-called critical density), it, in turn, determines the geometry of our universe.
If Ω>1, the universe would take the shape of a sphere (closed universe); if Ω<1, the universe would look like a saddle (open universe); if Ω=1, the universe would be deprived of any curvature and appear to be flat. Many measurements of the relative density have taken place, and the current density of the universe is observed to be very close to 1.
In 1980 to solve the horizon (another great conundrum of physics) and flatness problems, astrophysicist Alan Guth proposed the inflation theory which was later optimized by many others. This model, which suggests that the early universal expansion accelerated at a rate much faster than we see today, helps to explain why our universe is flat.
Source: Physics Girl via Youtube
Feature image credit: NASA