This is probably a stupid question with a very simple answer but I hope someone here can help me, I'll try to explain myself as best I can... I was thinking about spacetime being elastic, if I place a large mass on the elastic sheet it bends around it, when i place a second mass on the sheet, as well as the sheet bending around the second mass doesn't this slightly impact on the effect of the first mass? I was thinking as you continue to add different masses their effect on each other locally becomes less. I guess I was thinking that this describes the movements in spiral galaxies where masses move slower in the centre where the areas are more densely occupied. As an extra bonus, if the mass of the large black hole at the centre of our galaxy was estimated by measuring the speeds of stars orbiting it then this would also mean that the size had been massively underestimated which would help solve the problems of closely orbiting galaxies and the extra gravitational lensing effects that are all attributed to Dark Matter at the moment. Again this probably has a very simple answer and I apologise if this is below the usual level here but I'm struggling to answer this myself.