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KurtLudwig

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- 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.