During the formulation of the theory called "redshift" it was found that some spiral nebulae, when moving away from the earth, established a red shift.
By observing the redshift, the scientists were able to perceive that the nebulae were axially distant, that is, the Doppler effect occurred, where enlargement or reduction of the frequency of a wave could be observed in reference to the nebula in motion. From that, it was possible to conclude that the redshift is caused by the recession movement of the nebulae. Which may indicate that the universe is expanding continuously.