If there is no dark matter, in order to explain our observations, our concepts of an important building

block of the universe have to be incorrect. What is that building block and why do Astronomers
overwhelmingly favor dark matter, which they cannot see, to explain our observations, rather than the
building block being incorrect?

The important building block being referred to here is ordinary matter, which consists of atoms and subatomic particles that make up everything we can see and interact with in the universe. Ordinary matter alone, however, is not sufficient to explain the observations in astronomy.

Astronomers overwhelmingly favor the existence of dark matter primarily because of its gravitational influence on visible matter and its ability to explain various observational phenomena. Here's why:

1. Galactic Rotation Curves: When astronomers measure the rotation speeds of stars and gas in galaxies, they find that the outer regions rotate faster than can be explained by the visible matter alone. Dark matter provides the additional gravitational pull needed to keep galaxies intact.

2. Galaxy Clusters: The motion of galaxies within clusters suggests the presence of significant amounts of unseen matter. Dark matter acts as a gravitational glue, holding clusters together against the forces that would otherwise cause them to disperse.

3. Cosmic Microwave Background (CMB): Observations of the CMB, the afterglow of the Big Bang, provide strong evidence that the universe contains more matter than visible objects can account for. Dark matter is thought to compose about 85% of the total matter content of the universe.

4. Large-Scale Structure Formation: The distribution of galaxies and the formation of cosmic structures, like galaxy clusters and filaments, can be explained by the gravitational effects of dark matter. Its presence allows for the formation of structures we observe in the universe.

Despite its gravitational influence, dark matter has not been directly detected through traditional methods. However, there is substantial indirect evidence supporting its existence. Astronomers use a variety of observational techniques, such as gravitational lensing and simulations of the universe's large-scale structure, to study the behavior and distribution of dark matter.

In conclusion, astronomers favor the existence of dark matter because it provides the best explanation for various observational discrepancies in the universe. While there are ongoing efforts to understand it more comprehensively, dark matter remains an essential concept to reconcile our current understanding of the universe with the observations we make.