Well, we do have the ability to detect individual neutrinos, through their interactions with ordinary matter. That's not currently the case for dark matter: it's sort of in the place neutrinos were in between 1930 (when neutrinos were postulated (as you describe) to explain missing momentum) and 1956 (when they were first observed [0] in nuclear fission reactors).

https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experime...

/tangent I learned a fun fact! The original proposal for the neutrino experiment wanted to detonate a nuclear bomb. The reactor was a last-minute change.

- "Eminent physicists enthused about the plan. It was approved by Reines’s employer, the government-funded Los Alamos laboratory in New Mexico. Work began on the detector, nicknamed El Monstro, and on the construction of the shaft. At the last minute, Reines and Cowan transferred the experiment to a nuclear reactor, but not because of environmental or safety concerns. They had worked out that although the reactor would deliver a flux of neutrinos three orders of magnitude lower than that from the bomb, it offered a better option for distinguishing signal from noise."

https://www.physics.uci.edu/node/14126 ("The singing UCI Nobel Laureate who nearly bombed Nevada")