Centauri's Thoughts

Looks like my thought on this was right after all:

Here's the thinking:

Newton's laws of physics explain why our solar system stays together. But the planets are negligible in the overall gravitational scheme, with the Sun being the total ruler and containing 99.86 percent of all the mass.

The same Newtonian physics were long ago applied to galaxies, and the rotation of stars couldn't be explained, so dark matter was invented to make theory work.

But a galaxy is much different than the solar system, Cooperstock explains. The conglomeration of all the matter -- stars, black holes, gas, and dust -- is collectively the source of the galactic gravity. Even a black hole at a galaxy's center typically packs less than 1 percent of the galaxy's overall mass.

The overall galaxy's gravity "feeds its own motion ... unlike the case of the solar system," Cooperstock told SPACE.com.

In fact, I was just talking to a friend the other night about these motions, and the fact that the closer to the center you are, the more mass is to the outside of the galaxy, relatively.

The whole article on space.com.