Since two articles by Fritz Zwicky in 1936 and 1938, physicists know that there are not enough visible stars in galaxies to explain galaxy clusters staying together given the speed of galaxies moving relatively to each other. That's why Fritz Zwicky called the missing mass "dark matter", because it apparently was not bright enough to be visible to us and could only be detected by its gravitational effects on galaxies staying together.
Vera Rubin in the 1960ies measured the speed of stars in the outer regions of the Andromeda galaxy and found out that they all seem to have the same speed on their orbit of about 200 kilometer per second, independent of their distance to Andromeda's center, while Kepler's Third Law was postulating a slowing down of speed proportional to the square root of distance. Later measurements showed the same effect on all galaxies measured.
When the Cosmic Microwave Background was detected also in the 1960ies, its small fluctuations allowed for the calculation of the amount of nucleons generated during the Big Bang, which gives an upper limit on the total mass of stars, neutron stars, white dwarfs, planets, dust and clouds in the whole universe. Since a few years, astronomers have perfected ways of detecting this type of matter in the Universe, and they can account for all the nucleonic (or baryonic) mass generated in the Big Bang.
Since the start of the Hubble Space Telescope, we can see gravitational lensing effects throughout the Universe, and now we have a tool to measure the distribution of mass in the universe by the gravitational effects of galaxies and galaxy clusters.
And now we have two sets of measurement: We can measure visible matter a.k.a. baryonic matter at all temperature scales, and we know how much of it is out there. And we know how much gravitational effect is out there by measuring galaxy rotation curves, movements within galaxy clusters and gravitational lenses. And in all cases, the result is that there is only about 17% of visible matter compared to gravitationally detectable matter, which means that 83% of all matter is unaccounted for. So either we don't understand electromagnetism (and we can measure electromagnetic effects with 12 positions, and calculate it with the same precision), or we don't understand Gravity (and we are able to maneuver space crafts around massive bodies and get them exactly where we want them to be, and we can compensate for the relativistic effects of the gravitational field of Earth to measure positions on Earth to a few meter), or there is something out there which is massive, but not detectable by electromagnetic waves.
It's your choice. But it's not wild bullshit.