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The equations could be wrong (that is always a possibility in an system) but, if I have the time line correct, it was noticed that angular velocities of galaxies and clusters did not match the amount of luminous material that we could observe, it was posited that a form of matter was around/about galaxies that was not interacting through emr but was gravitationally (and perhaps weakly). Subsequent to this it was realised that this matter should not only help our equations of motion work out, we should be able to detect it because of its gravitational lensing effect; this was born out in observation.