>the universe was in an infinitesimally small, infinitely dense ball. Just sitting there. Now. Why didn't it stay that way?
See, this guy called Newton had this law which we commonly call "intertia" which says that things in motion remain in motion unless acted upon by an outside force and things not in motion remain motionless unless acted upon by an outside force.
Newton's laws are just macro-scale simplifications of other laws. They do not work at an atomic level, and anyway the laws need not be the same under the extremely special conditions that a singularity provide.
Anyway, proposing a "god" or any such thing in no way solves the problem of where the universe came from - it only pushes the problem one step further, and introduces a heap of unanswerable questions.
Rest assured that science has no need for the hypothesis of a god. It has never solved anything so far and the places where you can try to pry it in are getting fewer and fewer. See for example
God of the Gaps.