What evidence supports the theory of an expanding universe?

Evidence supporting the theory of an expanding universe includes redshift observations, cosmic microwave background radiation, and the distribution of galaxies.

One of the primary pieces of evidence for an expanding universe is the redshift of light from distant galaxies. When we observe the light from these galaxies, we find that it is shifted towards the red end of the spectrum. This is known as redshift and it's similar to the Doppler effect, where a sound's pitch changes as its source moves towards or away from you. In the case of light, if an object is moving away from us, the light it emits appears more red. The fact that we observe a redshift from almost all galaxies suggests that they are moving away from us, indicating that the universe is expanding.

Another key piece of evidence is the cosmic microwave background radiation (CMBR). This is a faint glow of radiation that fills the universe, and is thought to be the afterglow of the Big Bang. The existence of the CMBR was predicted by the Big Bang theory, and its discovery in 1965 provided strong support for this theory. The CMBR also provides evidence for an expanding universe, as the radiation is uniformly distributed in all directions, suggesting that the universe has been expanding and cooling since the Big Bang.

Lastly, the distribution of galaxies also supports the theory of an expanding universe. Observations show that galaxies are not randomly distributed throughout the universe, but are clustered together in groups and superclusters. This is consistent with the idea that the universe began as a hot, dense state and has been expanding and cooling over time, allowing matter to clump together under the influence of gravity. The large-scale structure of the universe, with its clusters and voids, is best explained by an expanding universe model.