Not even that. A theory is formulated after experimentation reveals certain observable behavior(s). And then the experiment is repeated.
By their own words, cosmologists and astro-physicists say dark matter is invisible and its effect on 'normal' matter is unpredictable.
Thus, 'dark' matter and 'dark' energy are nothing more that mathematical constructs that help them explain why their equations don't work. Sort of a "plugin" made of whole cloth.
As a Physics Major, the thing that concerned me most was the almost universal use of “Constants” that had to be used to get results that agreed with observed results.
I always considered a “constant” an excuse for lack of knowledge.