How can we prove the universe is expanding?

According to observations, the majority of the universe shows a red shift in the spectrum. When an object moves away from an observed, the waves are spread out further, and as the waves spread out, the wavelength lengthens and shifts the spectrum toward the red side of the spectrum.

The universe is expanding, as indicated by the red shift, which can be explained by the doppler effect.

The notion that the universe was eternal and had always existed in its current state was the accepted theory prior to Hubble's observations of the red shift, which suggests that the universe is expanding. Hubble's observations of the red shift, which implies an expanding universe, destroyed the steady state universe theory.

Numerous lines of evidence, such as measurements of the universe's large-scale structure, the redshift of galaxies, and observations of the cosmic microwave background radiation, all point to the expansion of the universe.