The concept of "prior to the Big Bang" is not well-defined within our current understanding of the universe. The Big Bang is considered the starting point of our universe, and conventional models do not describe any events or entities existing before it. Therefore, the statement that galaxies existed "prior to the Big Bang" is not supported by current scientific knowledge.
Regarding the discovery of mature galaxies, it's important to clarify the context. If you are referring to a hypothetical scenario in which six mature galaxies were discovered with age estimates that precede the age of the universe as we understand it, it would challenge our current understanding of cosmic evolution. Such a discovery would require a reassessment of our models and theories.
However, it is essential to be cautious when interpreting scientific findings and news reports. If you have specific information or a scientific publication in mind regarding the discovery of mature galaxies, I would need more details to provide a more accurate response.