cosmology - How can the universe be flat and have no center if universal mass-energy content is finite?

WMAP measurements confirm that the universe is flat within a 0.4% margin of error. If we assume the universe is flat and there is no 'center' then how could the mass energy content be finite since that would imply a cosmic boundary? (Boundaries define the location of the 'center')

*Let me be clear, I am not stating that there is an infinite mass energy content in the universe, I am simply taking advantage of the fact that the quantity of mass-energy content in our universe is still an open question.