> when you have a mass distribution that can be arbitrarily assigned in space, it feels like there are a lot of "nice things you could explain" Like an eight-p

> when you have a mass distribution that can be arbitrarily assigned in space, it feels like there are a lot of "nice things you could explain" Like an eight-planet star system, or a star system with a hot Jupiter, or a multi-star system with no planets, all arising within the same giant molecular cloud (GMC)? Even though the cloud-elements behave sanely, even small arbitrary changes in initial values can produce dramatically different later arrangements. Replace the galactic GMC with a dark matter filament in the cosmic web, and why be surprised that all sorts of galaxies form within? I don't think there is reason to fear arbitrary mass distributions so long as they obey constraints (if you're interested in the maths, the Einstein constraint equations are a good starting point you'll find in textbooks -- Robert Wald's chapter 10 is good and mathy, Alcubierre's or Baumgarte and Shapiro's numerical relativity textbooks are good alternatives for people who think computationally -- and lecture materials). We know empirical constraints and lots of solutions at many scales in the known universe already, and that constraint equations can be found for many other classical field theories; Dirac and successors (Regge et al., Rothe & Rothe et al.) are good resources wrt quantum field theories. Roughly the slogan is to lay down some arbitrary but plausibly generic configuration and see how it evolves. Do it often enough and you should get familiar structures, or you're back to the drawing board. Large cosmological dark matter simulations that do exactly this are new hat. Standard cosmology solutions often get compared in the same project with alternative theories, sometimes following the spirit of the Standard Model Extension in parametrizing everything possible and plugging in different coefficient values, sometimes simply trying to trace cosmological implementations of an IV formalism on some published alternative to general relativity. For example, Vogelsberger et al., <https://www.nature.com/articles/s42254-019-0127-2 https://www.nature.com/articles/s42254-019-0127-2>, <https://arxiv.org/abs/1909.07976v6 https://arxiv.org/abs/1909.07976v6> in which §5.3 has a brief literature review.