"Light matter" is basically stars. Now, sometimes bodies reflect light (which is why we can see the Moon or Neptune). Bu before the name "dark matter" existed

"Light matter" is basically stars. Now, sometimes bodies reflect light (which is why we can see the Moon or Neptune). Bu before the name "dark matter" existed, the practice of it was evident. How did they discover Neptune? Gravitationally, not by seeing it first via a telescope. Its existence was inferred by the orbits of other planets, namely a gravitational disturbance suggesting a big glob of matter we had not collected photons from. But we did not discover Neptune by collecting its photons. We had to infer that some glob of matter was present by gravitation. Then we pointed telescopes in the vicinity of prediction until this was confirmed. Going further, we use these collected to infer all kinds of things -- a consistent dimming of a star in a certain period might suggest a planet occluding the star in an orbit coplanar to ours, which is our current method of locating exoplanets, but that wouldn't count. You might recall how the neutrino was originally simply a placeholder, and actual detection came later. Similarly, the coining of the phase dark matter was simply as a contrast and a placeholder: we know something is there, but it ain't stars (light matter). Historically, many suggestions were made, and quite a few of them weren't exotic: MACHOs (Massive Compact Halo Objects), intergalactic gas, and so on. WIMPs would classify as a candidate for dark matter that would be somewhat exotic, so would axions. Frustratingly, as the article suggests, we can do a lot of things to rule out (or more typically, set a maximum contribution as a fraction from each candidate), but we have little ruled in. We don't even have detection of axions yet, and those have been in the running for decades. This is the sort of stuff that lured me into getting a physics degree.