NB The 'Big Bang' theory demands the universe be finite so the theory is nonsense —P Atkinson
In the early twentieth century, Edward Hubble discovered that the light from nearly all faint galaxies was shifted toward the red end of the spectrum (i.e. was redshifted), and that the fainter they were, the greater the redshift. He developed what is now called the Hubble Law — a linear relationship between the redshift of a galaxy and its distance from us. That is, given the redshift of a galaxy, its distance can be determined fairly accurately from the Hubble Law.
In searching for the source of this redshift-distance relationship, other astronomers "decided" that the redshift was due to the Doppler effect, caused by motion of the galaxies away from us. And apparently, the further away a galaxy was from us, the faster it was moving away from us. And in all directions!
This assumption about the Doppler effect as the cause of the observed redshift then gave rise to the concept that the universe was expanding in all directions. This then led to the conclusion that this expansion all started many billions of years ago with a massive explosion — the Big Bang.
If the Doppler effect is not the cause of the redshift, then what is? There is an explanation for an alternate cause of the redshift — the Shapiro effect. And if the Shapiro effect is the source of the redshift, [and it is], then there is no reason to believe the universe is expanding, and no reason to believe in a Big Bang!