The evidence that the universe is expanding comes with something called the red shift<span> of light. Light travels to Earth from other galaxies. As the light from that galaxy gets closer to Earth, the distance between Earth and the galaxy increases, which causes the wavelength of that light to get longer.</span>
When the light coming from any object is seen through a prism or diffraction grating, we can see its spectrum consisting of 7 colors (VIBGYOR).
When the object moves towards the observer the spectrum will shift towards the blue part (decrease in wavelength) known as Blue shift and if the object moves away, the spectrum will shift towards red (increase in wavelength) known as Red shift.
Edwin Hubble, Alexander Friedmann, and Georges Lemaitre in 1920s used observed Red Shift in the celestial objects to prove that galaxies are moving away which is in line with the Big Bang theory which states that Universe started with a huge expansion and is continuously expanding. Later on observation done through Hubble Space telescope corroborated this.
