1. Early Hypotheses and Observations

The hypothesis of dark matter has a rich and intricate history. The concept was first alluded to in the late 19th century. In the appendices of his book, "Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light," based on lectures given in 1884, Lord Kelvin speculated on the possible number of stars surrounding the Sun. Using the observed velocity dispersion of nearby stars and assuming the Sun's age to be between 20 and 100 million years, Kelvin considered the scenario where there were a billion stars within one kiloparsec of the Sun. He concluded that many of these stars could be dark bodies, unseen and unaccounted for in visible observations.

In 1906, Henri Poincaré, in his work "The Milky Way and Theory of Gases," used the term "matière obscure" (dark matter) while discussing Kelvin's ideas. Poincaré suggested that the amount of this dark matter needed to be less than the amount of visible matter to account for the observed dynamics of the Milky Way.

2. Developments in the Early 20th Century

The idea of dark matter continued to develop in the early 20th century. In 1922, Dutch astronomer Jacobus Kapteyn proposed the existence of dark matter based on stellar velocities. Shortly thereafter, in 1930, Swedish astronomer Knut Lundmark recognized that the universe must contain much more mass than could be observed directly. Dutch astronomer Jan Oort also hypothesized the existence of dark matter in 1932 while studying the motions of stars in the local galactic neighborhood. He found that the mass in the galactic plane must be greater than what was visible, although this measurement was later determined to be inaccurate.

3. Fritz Zwicky and the Coma Cluster

The most significant early evidence for dark matter came in 1933 from Swiss astrophysicist Fritz Zwicky, who was studying galaxy clusters at the California Institute of Technology. Zwicky applied the virial theorem to the Coma Cluster and found evidence of a substantial amount of unseen mass, which he termed "dunkle Materie" (dark matter). By estimating the mass of the cluster based on the motions of galaxies near its edge and comparing it to the mass estimated from the cluster's brightness and number of galaxies, Zwicky concluded that the cluster had about 400 times more mass than was observable. The gravitational effects of the visible galaxies alone were insufficient to account for the high orbital velocities, suggesting the presence of a large amount of unseen mass.

Although Zwicky's calculations were later found to be off by more than an order of magnitude due to an outdated value of the Hubble constant, his basic conclusion that most of the mass in the universe is dark was correct. This groundbreaking work laid the foundation for the modern understanding of dark matter.

4. Galaxy Rotation Curves and Further Evidence

Further evidence for dark matter emerged from studies of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve of the Andromeda Galaxy, which indicated that the mass-to-luminosity ratio increased with distance from the galactic center. Babcock initially attributed this to either light absorption within the galaxy or modified dynamics in the outer regions, rather than to missing mass.

In 1940, Jan Oort observed and wrote about the large non-visible halo surrounding the galaxy NGC 3115. This observation, along with Babcock's findings, suggested that galaxies contained a significant amount of unseen mass, further supporting the dark matter hypothesis.

5. Vera Rubin and the Modern Era

The modern era of dark matter research began in the 1970s with the work of Vera Rubin and Kent Ford. Their study of the rotational curves of spiral galaxies provided compelling evidence for dark matter. Rubin and Ford found that the outer regions of these galaxies rotated at unexpected speeds, with rotational velocities remaining constant or even increasing with distance from the galactic center. This was contrary to the predictions of Newtonian mechanics, which suggested that rotational velocities should decrease with distance. These observations indicated the presence of a significant amount of unseen mass distributed throughout the galaxies, now attributed to dark matter.

6. Gravitational Lensing and the Bullet Cluster

Gravitational lensing has provided further robust evidence for dark matter. This phenomenon occurs when the gravitational field of a massive object, such as a galaxy cluster, bends the light from a more distant object. Observations of gravitational lensing have shown that the mass inferred from the lensing effect often exceeds the visible mass, indicating the presence of dark matter.

One of the most striking examples is the Bullet Cluster, where two galaxy clusters have collided. Observations of the Bullet Cluster show a clear separation between the normal matter, detected through X-ray emissions, and the dark matter, inferred from gravitational lensing. This separation provides strong evidence for the existence of dark matter and its distinction from ordinary matter.