May I suggest a theory, not even much of a theory as of scientific fact! That galaxies are held in form by some kind of unkown matter. It's illusionary to call this matter "dark matter" when substantium has much more scientific basis for existence. Of course the force in the center of galaxies cannot be as powerful as substantium on earth, because we know and science tells us that earth is in the center of all. We even see galaxies being pulled towards us!
So, the force that holds galaxies together might be Substantium's little brother, Subtum, which we have all the reason to believe exists. It is not even close to as powerful as Substantium but powerful enough to keep galaxies in together.
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