Universe - I don't really remember what I did, however some things:
force net should not be summed, it should be recalculated each step
acceleration should not be summed, it should be a scaling of the force
net
force net should not be calculated from an object onto itself

main - added some initial velocities to debug, it seems that the zooming
glitch happens when the left corners of the entities intersect, will add
some code to check this behavior