How do astronomers know our universe is expanding?
In the 1930s, Edwin Hubble, an astronomer, detected hundreds of galaxies besides our own and some were very distant. He found that the light that reached Earth was mostly in the red part of the section. Thus, the REDSHIFT was discovered
The farther the object is, the more redshift its light will be. Hubble saw that these galaxies were moving away from Earth and their light was in the red part of the spectrum. Then he theorized that the Universe was expanding, which is proven to be correct.