By that I mean that the edges of galaxies experience a "reference" spacetime (e.g. constant period), while the interiors experience a stretched out version spacetime (i.e. inverse time dilation and length constriction)?
Or did the entire community of astrophysics simply ignore high school physics in favor or something vastly more complicated because reasons?
In physics, how well your model can predict past and future states from the current state is rather important. Though future states more than past state but any model should be able to also somewhat agree with it's the past it generates.
You can't simply say "this model also predicts that" without giving on how accurate it does predict and comparing to existing models. Otherwise we'd be using flat earth models for building bridges and planning ship routes.
You'd do well to do a little more research on the topic before posting so many comments on HN.
There is a ton of evidence for dark matter. We still don't know exactly what it is, but we do know lots about it (e.g. it is not baryonic matter because observation differs from theoretical models of how it would interact with ordinary baryonic matter). As for all that other evidence, the first hit on Google is for laymen: https://medium.com/starts-with-a-bang/five-reasons-we-think-...
TLDR: gravitational lensing, missing mass, and three other really good reasons that dark matter is a serious subject of study.
Disclaimer: I am not a physicist. And I'm trying not to represent myself as one on HN.
Talking about this stuff is fun, sure. It's great to learn! But please don't spread ignorance with such high confidence. As for "just" Newtonian physics, there's a whole research field called "Modified Newtonian Dynamics" (MOND). https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
TLDR; there are lots of problems with MOND, and it doesn't explain gravitational lensing and other effects of dark matter that we see.