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The planets and the Sun are likely in stable orbits because they both attract and repel each other with the same force.
The opposite of gravity is hilarity.
Unfortunately (perhaps fortunately) there is no such force.
Surely it is two words. Dark energy.
maybe the universe is populated with gravitons and antigravitons and we just don't see the latter very often?
Anti-gravity is science fiction isn't it.
It seems to me that it is the repulsive force that is expanding the universe and that is the opposite of gravity.
But it don't have a name!
We can measure gravitational attraction at a mesoscopic level, in the laboratory, geologically, and by making orbital calculations. Dark energy doesn't seem to invade our calculations - any such contribution must be orders of magnitude less than gravitation (though it must be admitted that G is probably the least precisely known of the fundamental constants). This makes it rather different from the common forces and antiparticles, which generally mirror one another exactly.
Except of course that positrons and antiprotons, whilst being exact inverse analogs, are very rare.
We have so with negative mass we get so that the sign of the force is reversed. This would result in a repulsive force. Whether or not negative mass has any realistic meaning is a matter of philosophical debate.
It could simply be a mathematical convenience that produces the correct results. Define mass in definitive terms. It is like saying define energy in definitive terms.
Maybe we could call the expansionary force the X-force.