Final answer:
Observations of galaxies moving toward each other or evidence that galaxies could not have formed as early as predicted by the Big Bang theory due to large density fluctuations would disprove key predictions of the Big Bang theory.
Step-by-step explanation:
Any observation that galaxies are moving toward each other would contradict the Big Bang theory, which predicts that galaxies are moving away from each other as evidenced by the red shift. A key piece of evidence supporting the Big Bang theory is the cosmic microwave background radiation, which presents as a perfect blackbody spectrum characteristic of the post-Big Bang universe. Additionally, another discrepancy would be if the density fluctuations were too large to have allowed galaxy formation in the time frame that is estimated by the Big Bang theory, given that energetic photons would have inhibited such early galaxy formation by interacting with hydrogen and helium. Thus, these observations would have a substantial effect on the Big Bang theory's validity.