I Does gravity act differently on a Solar System, a Galasy, and the Universe?

[KurtLudwig]

TL;DR Summary

Newton's law predicts gravity for our solar system almost perfectly. On a galactic scale, modified Newtonian dynamic equations predict the stellar rotation curves. On the scale of our Universe, black matter is needed to explain large scale phenomenon.

Since acceleration due to Newton's law decreases as the inverse of distance, it becomes very weak at large distances. Our Sun was unable to pull in matter in the Oort cloud in over 4 billion years. Above about 0.11 light years, using modified Newtonian dynamics equations, gravity decreases by the inverse of distance only. Gravity at that distance from the sun is about 1.2 x 10^-10 m/s^2. That is, gravity decreases much more slowly. On a galactic scale, there is no need to assume dark matter. However, to explain formation of stars in the beginning of our Universe, gravitational wells of dark matter were needed. Other very large scale phenomenon can be best explained by existence of dark matter.

 

[PeroK]

Is there a question here?

 

[Janus]

modified Newtonian dynamic equations predict the stellar rotation curves.

Not so much. Recent observations have detected galaxies that have rotation curves that are close what Newtonian Physics predicts, while other very similar galaxies have rotation curves that differ significantly from the Newtonian norm. If MOND was the sole answer to galaxy rotation curves, it would have to be consistent from galaxy to galaxy; similar galaxies would have to have similar rotation curves. The newly discovered galaxies don't adhere to this pattern.

 

[Nik_2213]

MOND is a contender, but seems to pose more questions than answers, and doesn't seem to apply evenly. For every example of galaxy or galaxy group that 'needs' MOND, there seems a counter example that doesn't. Plus a couple 'in between'.

Like 'Dark Matter', or loathe it, there's wide gaps in our census of galactic stuff. How common are 'Intermediate Mass Black Holes' ? Finding a significant population of them would fix one big gap. Finding a lot more 'sub-Brown Dwarf' L/T/Y types in our solar neighbourhood would shift the 'mass function'. Assuming, if we dare, that our location within an 'interstellar bubble' is even mildly representative of wider 'Spiral Arm'...
https://en.wikipedia.org/wiki/Local_Bubble

So, beyond the usual astro-sites, I keep a wary eye on RECONS: (REsearch Consortium On Nearby Stars) "...To understand the nature of the Sun's nearest stellar neighbors, both individually and as a population. Our primary goals are to discover "missing" members of the stellar sample within 10 parsecs (32.6 light years), and to characterize all stars and their environments within that distance limit. "
http://www.recons.org/

 

[KurtLudwig]

The question was implied: Can both dark matter and modified Newtonian dynamics help us explain the clumping of matter, gravitational lensing and galaxy rotation curves. Does it have to be either or? Can it be both? Phenomenon of light are explained by waves and particles.
The ideal gas laws, which need to be sometimes modified due to Van der Waals forces between molecules of some gases. Maybe astronomers are missing a gravitation phenomenon between stars?
I do defer to Janus' knowledge on astronomy. Please take this post as questions. I find astronomy very interesting and am amazed at the insights discovered during the last twenty years and during the last century.