I am not a real physicist, so please bear with me if my question sounds stupid to all experts around here. What I have learned about the dark matter is, that objects behave as if there was more mass than actually visible. So, there's a discrepancy between the "measured" mass and the observed movement.
As an alternative to assume dark matter being responsible for this observation, one cold also postulate that the gravitational constant G might be different in other locations in the universe or around big masses, or may be dependent on other factors like the mass itself or some unknown mechanism.
So, why does nobody take this into consideration? Does G have to be constant in the universe? Would there be side-effects that make this assumption impossible? Is there any prove or evidence that G is constant? Why do we assume that? Would a different value for G or a non-constant G be able to produce effects similar to those we see? I guess, that I am neither the first one with this idea nor that this question is unanswered yet. It's just I didn't find the answer and don't know where to look for it or whom to ask. So, if anyone of you guys could briefly tell me why this idea won't work.
Thanks in advance! Best TomS