To quote http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/, "It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. " I know (roughly) where these estimates come from, but I don't see how we quantify them. My best guess is that viewing the universe as a spacial 3 sphere (ie everywhere similar) with time running at the same rate everywhere, we can estimate the amount of 'ordinary' matter per unit volume, assuming pressure is a negligible, and we can estimate the 'mass equivalence' of any dark matter (ie the extra mass suggested by the speed of rotation of galaxies), and then the 'negative mass equivalence' suggested by the rate of acceleration of expansion of the universe. Is that roughly how the percentages are calculated? Is there an authoritative source to quote?