http://scitechdaily.com/astronomers -measure-the-temperature

If we were wrong about the mass of the universe both dark matter and dark energy would change in quantity. Dark matter has a considerable amount of non-cosmological evidence behind it. So most likely it would stay in some new quantity (since dark matter is the largest component of the universe in mass). Dark energy's future would be more uncertain however. To be wrong about the mass of the universe tests of cosmology would have to be wrong, it is not simply assumed. If these tests were wrong then you couldn't trust the dark energy result until you rectified the problem.

The temperature of the universe (as in the CMB if that's what you mean) is measured not assumed. If that was wrong the CMB results (the most powerful test in cosmology) would be uncertain. If we were wrong about the temperature low mass dark matter like neutrinos could make a comeback.