Final answer:
Dark matter is essential in cosmological models, explaining the gravitational forces in the universe not accounted for by visible matter. It's crucial for galaxy formation and is supported by evidence such as gravitational lensing and the critical density balance.
Step-by-step explanation:
Astronomers believe in the existence of dark matter because it appears to be necessary for explaining the structure and evolution of galaxies as observed in the universe. The Big Bang theory and current cosmological models rely on the presence of dark matter to account for the gravitational effects that cannot be explained by visible matter alone. Evidence from gravitational lensing and the Cosmic Microwave Background (CMB) implies that there must be a significant amount of matter that does not consist of atoms with protons and neutrons. This non-baryonic form of dark matter is thought to have played a crucial role in the formation of galaxies in the early universe by serving as an underlying gravitational framework.
The precise balancing of the universe's density around the critical density is a key issue addressed by the introduction of dark matter. Without it, the universe's expansion could have been too rapid to allow galaxy formation or too slow, leading to a universe that would have begun contracting before galaxies had time to form.
Further supporting the existence of dark matter, observations reveal the phenomenon of gravitational lensing, where light from distant objects is deflected in a way that indicates the presence of a large amount of unseen mass. These observations support the conclusion that dark matter is an essential component of the universe, outweighing visible matter and playing a vital role in its structure and expansion.