The universe is vast, filled with mysteries that challenge our understanding of reality. Among these, dark matter stands out as one of the most elusive and significant components of the cosmos. It is an invisible force that cannot be seen directly, yet it exerts a profound influence on the structure and behavior of galaxies, stars, and the universe as a whole. This chapter delves into the concept of dark matter, exploring its nature, how it was first theorized, and why it remains one of the most intriguing puzzles in modern physics and astronomy.

Dark matter was first proposed as a solution to an anomaly in the way galaxies rotate. In the 1930s, astronomer Fritz Zwicky observed that galaxies in the Coma Cluster were moving much faster than expected, based on the visible matter in those galaxies. He suggested the existence of an unseen mass, which he called "dark matter," to account for the gravitational pull that would explain these high speeds. This groundbreaking idea, however, was met with skepticism at the time. Over the decades that followed, evidence continued to accumulate, pointing to the presence of dark matter in the universe.

Dark matter is not something we can detect with traditional instruments because it does not emit, absorb, or reflect light. It is essentially invisible to electromagnetic radiation, which makes it very different from the ordinary matter that makes up stars, planets, and everything we can see in the universe. Despite its invisibility, dark matter's presence is inferred through its gravitational effects on visible matter. For example, when we observe the rotation of galaxies, the stars at the outer edges are moving faster than they should, based on the amount of visible matter in the galaxy. This suggests that there is additional mass, unseen but detectable through its gravitational pull, holding these galaxies together.