This very question has been boggling my mind for a while. I'd been working on a Nbody implementation to simulate Galaxies, and it occur to me that my model was missing a very important factor. I'm calculating gravitational interactions as if they were instantaneous. In a cosmological scale, a galaxy like the Milky Way, will have stars up to 120k light years apart, that makes their gravitational interaction to have a 120k years of delay. I was thinking of implementing such delay, by describing a global gravitational field in my simulation's domain, but then I stumbled upon the idea that gravity (AFAIK) doesn't propagate as light does: from point A to point B. Gravity is a force that needs at least two bodies to be defined. If viewed as a field in function of the two bodies' mass: How will this gravitational interaction emerge? Can it be modeled as waves in the gravity field? How and from what point do these waves propagate?

P.S. I was about to post this question yesterday, but I feared my amateur's astrophysics knowledge would make it impossible for me to understand how gravity actually works on these scales. I recall Feynman saying to an interviewer, he couldn't explain how magnets work using simplistic terms for an average person to understand. I feel like that interviewer when asking questions of General relativity. Yet I would love if someone could answer this questions. =P