First of all, how is this unobservable matter more valid than an unobservable god?
Because of indirect empirical observations?
To decide which is more valid we would need to set out the grounding of each claim. I have never heard of a theist saying "the god I believe in is hypothetical". Scientists stamp dark matter as being hypothetical, they do not insist it is true, just a plausible explination worthy of further research.
Second, does the fact that dark matter has been produced as a way for scientists to maintain their core beliefs (how gravity works) a sign that they are somewhat fundamental about their beliefs? Ruse in "Creation science is not a science" says scientists are not fundamental about their beliefs. (And creation scientists are fundamental)
I think (good) scientists are well adapted to abandoning beliefs when faced with evidence to the contrary. Its the nature of science to improve on what is known.
Also, could this situation be like the outdated phlogiston theory? Scientists had an idea of how things burned, their tests didn't quite match up, so they adjusted their theory. Kuhn shows us how this is an example of a paradigm shift. Once we figured something else out (how oxygen works) we were able to shift paradigms. Could scientists be stuck in a paradigm about gravity and forces in outer space that will eventually shift (once we have a new/better theory)? Could dark matter just be buying us time, a way to make sense of things that we can't quite understand just yet?
It could very well be. Science is at a loss when it comes to trying to reconcile the behaviour of the very small with the behaviour of the very large.
General relativity has passed many experimental tests Tests of general relativity - Wikipedia, the free encyclopedia
and even if GR fails precision testing at the scale of galaxies (see link), it still works fine in its limited domain. Just as Newtons ideas of gravity works fine in its domain and was not abandoned when GR came along, GR wont be discarded either.
May I ask, why do you think people have yet to abandon certain ideas? For example why do people abandon phlogiston theory yet cling to the idea of god?