About a century ago, scientists began to speculate that there might be some other kind of matter in the universe. In 1922, the Dutch astronomer Jacobus Kapteyn became the very first person to ever suggest the existence of such a substance. Then, a decade later, Jan Oort developed a similar hypothesis. He studied the motions of the stars in neighboring galaxies and came to think that the mass in the galactic plane must be greater than what was observed. Although, later Oort turned out to have been wrong about that particular assumption.
The following year, a Swiss astrophysicist named Fritz Zwicky was working at the California Institute of Technology when he made a similar kind of inference. After applying a theorem to the galaxy cluster that he was studying, he obtained evidence of something he called “dark matter”. Zwicky estimated that the cluster had a few hundred times more mass than was visible to him. He knew that the gravitational effect of the galaxies was far too small for the relatively fast orbits he was observing, thus that mass must be hidden from his view somehow. So, he concluded that invisible objects provide the mass as well as the associated attraction to hold the cluster together. In this way, he formulated the first formal hypothesis regarding the existence of dark matter.
Of course, just as Oort had made some miscalculations, so too did Zwicky. His estimates were off by more than an order of magnitude, due to an obsolete value of the Hubble constant that was being used at the time. However, Zwicky did correctly infer that the bulk of the matter was dark, meaning non-luminous. In 1939, Horace Babcock even went so far as to declare that the rotation curve for the Andromeda Galaxy suggested that the mass-to-luminosity ratio increases radially. However, he was not a proponent of the dark matter hypothesis. Like most astronomers he was still very skeptical about the whole thing.
Nonetheless, that all changed when Vera Rubin used a spectrograph to measure the velocity curve of edge-on galaxies with a high degree of accuracy. The result of the findings was eventually confirmed in 1978 and published in 1980. It was then determined that most galaxies must contain about six times as much invisible as visible matter. Thus, the apparent need for dark matter became widely recognized as a major unsolved problem in science. Following this, a series of observations in the 80's indicated the presence of dark matter in the universe, including the pattern of anisotropies in the cosmic microwave background radiation, the temperature distribution of hot gas within galaxies, and the gravitational lensing of distant objects.
To summarize, dark matter only interacts with certain particles, like gravitons but not photons. It doesn’t even interact with itself. This non-baryonic material also makes up about five-sixths of all the matter in the universe. In the most technical sense, matter is anything whose energy density scales with the inverse cube of the scale factor, in contrast to radiation which scales to the inverse fourth power. Thus, dark matter is a component of the universe which is not ordinary matter, but still obeys the same law. This has led cosmologists to believe that dark matter must be composed of an uncharacteristic particle. The search for this presumably low-velocity particle is one of the major efforts in contemporary physics, and it never ceases to amaze me how much can be known about something that can’t even be proven.
P.S. If you liked learning about dark matter, then you might also want to read my essay about dark energy. Here’s a link to it: