What force is responsible for the formation of the solar system from a nebula?

Answer 1

Gravity starts it. Small (relatively small, they are still along the lines of a lightyear in diameter) pockets of the cloud which are slightly denser then the rest (because the universe is not uniform, so parts will be denser then others), there is slightly more gravity pulling towards those dense pockets then other, less dense, pockets. As gravity pulls more dust and particles to these pockets, they get denser and thus have more gravity. This cycle continues for millions of years until the particles eventually get so hot and under so much pressure that the nuclear process begins and a star is born. Gravity then also makes planets in the same why, except, of course, they don't get large enough to start a nuclear reaction inside of them.