When scientists observed the light from stars and galaxies, they noticed that their color shifted toward the end of the visible color spectrum.
a. violet
b. green
c. red
d. blue

When we talk about the visible electromagnetic spectrum, we know it starts in violet-blue and ends in red.

Now, in this context the astronomer Edwin Powell Hubble observed several celestial bodies, and when obtaining the spectra of distant galaxies he observed that the spectral lines were displaced towards the red one (red shift), whereas the nearby stars showed a spectrum displaced to the blue one.

From there, Hubble deduced that the farther the galaxy is, the more redshifted it is in its spectrum, and noted that all galaxies are "moving away from each other with a speed that increases with distance".  

In other words: In the past the distance between two galaxies was smaller than at present, being this the proof that the universe is expanding (like a balloon expands when it is filled with air).

It should be noted that the redshift is not produced by the relative movement of the galaxies with each other.

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this means that the temperature of both substances will be equal