The redshift

Let's suppose that you are standing on the road way trying to hitchhike but unfortunately, you're a man so nobody stops to pick you and you just let cars pass nearby, but suddenly you saw a police car that is chasing another car and when they pass by you, you notice that the sound when the car is coming towards you seems high pitched and low pitched when the car is moving away from you. This is called the Doppler effect and it's explained by the shape of the wavelengths

The same paradox occurs within light waves :

The visible light is constituted from 7 different colors which are called the spectral colors and are graduated from  red to violet with red being the least energetic and violet the most energetic while the wavelengths are completely opposite due to the fact that the energy and the wavelength are inversely proportional according to the equation E=h*c/ λ  (E: energy, λ: wavelength, h,c: constants)

So by examining the previous figure, we could analogically assume that the light waves in the incoming direction seem to appear more bluish in the opposite side the outcoming waves seems to appear more reddish. This feature is universal and is one of the strongest indicators of the real existence of the big bang and the universe expansion.In fact, the further away a galaxy is, the faster it appears to be moving, and hence the more the spectrum is redshifted. Astronomers can, therefore, measure the distance to a galaxy by taking a spectrum and seeing how much the lines that you would expect to see have been shifted.