Imagine a certain rare galactic event A that occurs with probability 0.5 whenever a certain set of conditions are met

It seems like this is an indirect description of the Black Swan problem. Consider an astrophysical event so unlikely that it has a 50% chance of occurring in the entire light cone. Theory A completely prohibits the event, while theory B assigns the event a very small probability. If the event does not occur in the observation set, there is no way to distinguish between A and B.