
Originally Posted by
x(x-y)
I thought that gravity is asymptotic in the fact that it decreases away from the source, but never drops to zero. So, I don't quite understand how it can cause, essentially, an effect of "negative gravity" at large distances.
You are thinking of standard Newtonian gravity. There are versions of Newtonian gravity that people played around with that had repulsive forces at large distances, but they didn't amount to anything. Einstein revived this idea, somewhat, when he was trying to create a static, eternal universe model and added a term to the way gravity works in GR so that the farther away things are from each other, the more they, in a sense, feel a repulsive force from each other.
It is essentially impossible to balance out the repulsive forces of an entire universe with the attractive force of an entire universe, so even with this addition one cannot expect a universe without expansion or contraction.
One can run the math for this as either a part of how gravity just works or as part of how the energy associated with the vacuum couples with gravity. Either way, the graviton would be the relevant particle, if it exists.
There are a bunch of hypotheses of things that could be different but act a lot like dark energy. Lawrence Krauss wrote a book, Quintessence, about these. I have it, but I haven't read it.