It’s based on actual observations, with some assumptions on the nature of the universe. As you said - dark matter is detected by extra “holds stuff together” fo

It’s based on actual observations, with some assumptions on the nature of the universe. As you said - dark matter is detected by extra “holds stuff together” force than observed masses would indicate exists from a host of phenomena (find the Wikipedia article on dark matter for more detail), and dark energy which is the energy required to make the universe expand at an accelerated rate is based on on distance-redshift relation, measurements of the cosmic background radiation, and theoretical additional non matter or dark matter energy required to form an ‘observationally flat universe’. The basics for both of these are discussed on the Wikipedia for both subjects. Disclaimer - I am not a physicist but I did stay at the holiday inn express last night and I do watch a lot of PBS Spacetime on youtube.