You're also trying to explain Newton's Law of Universal Gravitation to someone with an Astrophysics degree.
"Solid system"? Funnily enough the Sun is not "solid", yet it is held together by it's own gravity.
The standard, accepted theory about the formation of black holes is that stars of mass >1.4 sol masses go supernova, compressing their core into something denser than neutrons and then "fall out" of regular space. Just being big won't do it.
Interestingly enough, as Hikaru pointed towards, if you use standard physics to try to model galactic rotation it fails. The galaxy collapses in on itself. And so physicists have suggested a modified model which isn't just dependent on the inverse square of the separation but also to the straight inverse of the separation.
Ops, I'm sorry Kryss to have posted my opinion. But whereas you have an astrohpysics degree or not, some facts cannot change in virtue of that. It is true that for a body to become something like a black hole, must have at least 1.44 masses like our sun's (the Chandrasekhar limit) but it does not point out that the body must be a sun. It consists of a gravitational field around a massive object, who collapses on itself due to it. It happens that in the universe, the most common objects for that are suns, (yeah, even I know that they are not solid in the common sense, they consist of H and He primarily)
Galaxies cannot collapse on themselves since they are not "solid" (they are not a single object, but a collection of billions of those, with their particular centre of gravity like suns and nebulae) The same objects who circle around it's massive core, with a really huge black hole, isn't it wonderfull? Really amazing! contrary to GL which was a single object way over the Chandrasekhar limit, with far more matter into it than a galaxy.
But of course, it gives an awesome show and no one stops to think about the imposibility of that while watching it trow galaxies and planets around. No one is trying to diminish GL. My comment was designed only as a silly comment. I guess I failed
Remember something: A dregree does not make ideas infalible. Those are only closer to the facts as we understand more and more about nature and our place in it. There are no autorities, only experts. All ideas are whorty of expression. Even the silly ones. Eve mine for that matter.
Peace
