Björn Malte Schäfer (Heidelberg) gives a talk at the Workshop on "Why trust a Theory?" (7-9 December, 2015) titled "Dark Gravity, Dark Fluids, and Dark Statistics". Abstract: Observational cosmology has the purpose of investigating gravity on large scales through the observation of the expansion dynamics of the Universe on one side and through cosmic structure formation on the other. In my talk I will go through the arguments why these observations are able to constrain models of gravity, what constraints there are independent from the assumptions of a certain model, how cosmology switches from being statistics to systematics dominated, and what precision and accuracy need to be archived by future surveys. Lastly, I’ll describe the process of statistical inference, errors, priors and fundamental limits in cosmology.