Dark matter is one of the most perplexing and elusive concepts in modern astrophysics. It was first proposed in the early 20th century when astronomers and physicists observed that galaxies and galaxy clusters seemed to be moving in ways that couldn't be explained by the visible matter alone. Despite the fact that dark matter cannot be directly seen or measured through conventional means, its existence has been inferred through its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.

The concept of dark matter emerged after astronomer Fritz Zwicky made an important observation in the 1930s. He noticed that galaxy clusters were moving much faster than they should have been based on the amount of visible matter present in them. In other words, there seemed to be an invisible force or substance that was exerting additional gravitational pull on these galaxies. However, it wasn't until the 1970s when Vera Rubin made similar findings in her studies of galactic rotation curves that the theory of dark matter gained significant traction.

Rubin's work revealed that the outer edges of galaxies were rotating faster than expected, based on the amount of visible matter present in them. If only the visible stars, gas, and dust were taken into account, the outer regions should have been moving slower, but they weren't. This discrepancy suggested that there was more mass in these galaxies than what could be seen. This unseen mass was later attributed to dark matter, a substance that doesn't emit, absorb, or reflect light, making it undetectable by conventional instruments.