The theory that the universe is expanding is supported by data from the

red shift of light from distant galaxies

Explanation:

Astronomers are able to monitor light from galaxies far away. When they do, they see that it's distinct from the Sun's light. The dark lines throughout the spectra are showing a rise in wavelength from distant galaxies.

The lines are transferred to the red end of the spectrum or changed.

This is called the Red-shift effect.

Astronomers in nearly all galaxies see the red-shift. This is a consequence of the growing vacuum between the Earth as well as the galaxies. This expansion spreads out all the light waves as they pass to us, moving them to the red side of the spectrum. The further the planet travels away from Earth the more red-shifted the light from a galaxy becomes.