Why does the presence of redshift in spectra of galaxies support an understanding of an expanding universe?

Doppler's Effect: If a source is moving towards the observer the frequency of the waves emitted by the source will increase (wavelength will decrease). If the source is moving away from the observer the frequency of the waves emitted by the source will decrease (wavelength will increase). The best example of this is the changing frequency of siren of an ambulance as it moves towards us and then away from us.

The same phenomenon can be observed in celestial objects. When the object will move towards us the wavelength will decrease thus the spectrum will be shifted towards the blue part. This is known as blue shift.

If the object moves away from us the wavelength will increase and thus the spectrum will be shifted towards red part. This is know as Red-shift. This is because red has longer wavelength (620 - 750nm) than blue (450-495nm)

Thus if a galaxy shows us red-shifted spectra it means that it is moving away from us. we have observed this phenomenon in various galaxies thus supporting the expanding universe theory.