Assuming gravity is matter curving space as Einstein says, isn't our theory of dark matter just an assumption that because more gravity is required to explain galaxy formation that it must be caused by unseen matter? Why do we assume that the curvature of space required must be caused by matter? Another question, using a slinky as an analogy. Everyone is familiar with stretching one and observing traveling pressure waves along it. Those pressure waves are like space curvatures caused by mass. But in a slinky the high pressure waves are also accompanied by low pressure waves to compensate, sort of an equal and opposite reaction. Has anyone ever thought that the curvature of space caused by gravity has a similar effect of less curved space near it to compensate? If that's true, wouldn't that act like dark energy?