A: In order to understand the validity of the Big Bang Theory, we would like to first understand what the red shift is. The red shift is a phenomenon wherein waveforms that move away from an observer experience an increased wavelength due to its motion relative to the observer. This is basically the Doppler’s Effect, wherein relative motion causes the frequency - and hence wavelength - to change in the observer’s perspective.
Now, when it comes to space, we’re essentially capturing the visible light waves (and possibly other electromagnetic waves), and upon receiving them, we realise that these observed wavelengths are somewhat shifted towards the “red side” - i.e. the region of the electromagnetic spectrum with longer wavelengths. This implies that these stars are moving away from us naturally. This corroborates the Big Bang Theory, which states that the universe actually expanded from an initial point of high density and temperature, and then has been expanding away at a very fast rate ever since.