Not sure I follow you (nor does it seem relevant), but "merely inferred {X} from the difference between calculations and observations" pretty much describes most of astronomy, doesn't it? Think of exo-solar planets (any method of detection), star-spots, spectroscopic binaries, the SZE, even all (radial) Doppler motions, magnetic fields, ...

[Edit: as the observational basis of DM is a popular topic, I have started a thread in the Q&A section - What is the observational basis for (cold, non-baryonic) dark matter?]