I am answering here because of the nesting limit.
If you assume the presence of dark matter in the universe, then you know its amount exactly. Cosmology allows you to make the calculations. You can fit the parameters of the cosmological model according to the latest data, and you get 26.8%.
This may change in the light of new data of course, as there are uncertainties. But give or take it will not be very different from this number (otherwise new data may contradict old data).