  In this paper, by analyzing the instability against spatially homogeneous and
anisotropic perturbations of the Kantowski-Sachs-type during different
cosmological epoch, we show that it is a theoretical consequence of the general
relativity that the KS universe must be open or flat if it underwent the matter
dominated and/or radiation dominated era in its past evolution, which
theoretically confirms the flatness of our observable universe.
