I would like this thread to explore the asociative effects of both dark energy and dark matter in the universe. I have recently had the priveledge of learning a lot of good information from the likes of SpaceTiger and Garth and would like to relate some of what I've learned to this topic. Initially, I would like to clarify some basic assumptions. I will start with some questions regarding orbital dynamics. A body in orbit essentially embodies Newton's law that a body in motion will stay in motion unless acted upon by an external force, right? It's orbit is determined by the gravity field in which it is orbiting, right? Does the gravity of the orbiting body effect the relative orbital relationship between it and the primary mass? That is, if Venus was replaced by another solar mass, would it still proceed around the sun in one venus year?