The redshift of light from most galaxies is evidence that the universe is expanding. As galaxies move away from us, their light is shifted toward the red end of the spectrum. This is because the stretching of space-time causes the wavelength of light to increase as it travels. The greater the distance to a galaxy, the greater the redshift of its light.

Edwin Hubble discovered the redshift of galaxies in the early 20th century. He found that the further away a galaxy is, the faster it is moving away from us. This observation led to the development of the Big Bang theory, which states that the universe began about 13.8 billion years ago in a hot, dense state and has been expanding ever since.

The redshift of light from galaxies is one of the most important pieces of evidence supporting the Big Bang theory. It provides strong evidence that the universe is expanding and that it has a finite age.