Redshift
Redshift
In the visible spectrum of light, in the image to the left you can see that towards the left is purple, and towards the right is red. The farther to the left a color is, the shorter its' wavelength is, the closer to the right a color is, the longer its' wavelength is. So if a source of light is far away from you, the waves will take longer getting to you and will be longer, and if its' wavelengths are long, they will appear red. if it is close to you, the shorter its' wavelength is, the shorter the wavelengths, the more purple or violet it will appear to you. So, if a source of light like a star is moving away from you, its wavelengths will increase and it will slide towards the right of the color spectrum and become closer to red.
Big Bang Theory
As we examine distant stars, we observe that most of them, over a course of time, slowly turn more red and go right on the spectrum of light. These observations and knowing Redshift makes us come to think that most stars around moving away from the center of the universe because they are changing colors to red which means that their wavelengths are increasing in distance.