Re: Hitting the wall
That natural force is made up of matter/particles (ionized and otherwise), velocity/force of matter, gravitational force, gas, radiation, magnetic fields (can have an effect on matter), just as it's plain to see in the ripples and arms of our Milky Way and other galaxies. Just because we can't see it, it doesn't mean that it's not playing a major role from the macro level on up and yes, vice versa.
Alex Blackwell posted some great links above. I thought I would just read one(read the pdf), but I ended up staying up for about two hours (Thanks, Alex).
Marcia