What role does dark matter play in galaxies?

Dark matter provides the gravitational force necessary to hold galaxies together and influence their rotation speed.

Dark matter is a mysterious and invisible substance that makes up about 27% of the universe. It doesn't interact with light or other forms of electromagnetic radiation, making it impossible to detect directly. However, its existence is inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.

In galaxies, dark matter plays a crucial role. It provides the extra gravitational force needed to hold the galaxies together. Without dark matter, the stars at the edge of a galaxy would move at a much slower speed than they actually do, due to the lack of visible matter present. However, observations show that stars at the edge of galaxies move at nearly the same speed as those near the centre, implying the presence of a large amount of unseen matter providing additional gravitational pull. This unseen matter is what we refer to as dark matter.

Furthermore, dark matter also influences the rotation speed of galaxies. The rotation curves of galaxies, which plot the speed of stars against their distance from the galaxy's centre, remain flat or even rise at large distances, instead of falling as would be expected if only visible matter were present. This again suggests the presence of dark matter.

In addition, dark matter is thought to play a key role in the formation and evolution of galaxies. The gravitational pull of dark matter is believed to have helped pull matter together to form galaxies in the early universe. Today, the distribution of dark matter continues to influence the structure and distribution of galaxies across the universe.

In summary, dark matter, while invisible and mysterious, is a fundamental component of galaxies. It provides the necessary gravitational force to hold galaxies together, influences their rotation speed, and plays a key role in their formation and evolution.