The flatness of the Universe can be determined through various measurements, such as the cosmic microwave background radiation, the large-scale distribution of galaxies, and the gravitational lensing effect. These measurements all point towards a flat geometry for the Universe.
A flat Universe means that the geometry of the Universe follows the rules of Euclidean geometry, where parallel lines never meet and the angles of a triangle add up to 180 degrees. This indicates that the Universe is infinite and has a constant curvature.
The flatness of the Universe is a prediction of the Big Bang theory. According to the theory, the Universe underwent a rapid period of expansion in its early stages, which resulted in a flat geometry. This prediction has been supported by various observations and measurements.
Currently, the Big Bang theory is the leading explanation for the flatness of the Universe. However, there are alternative theories, such as the Steady State theory, that propose a non-flat geometry for the Universe. However, these theories are not supported by as much evidence as the Big Bang theory.
If the Universe is truly flat, it suggests that it will continue to expand forever. This would mean that the Universe will eventually become a cold, dark, and empty place as all the stars and galaxies move further apart. However, there are still uncertainties and ongoing research about the exact fate of a flat Universe.