I have been puzzled by this for years, so I would welcome some enlightenment. It seems that Einstein was enamored with Mach's principle while searching for GR, but in the end GR does not seem compatible with it - or rather has nothing to say about it. What I mean is that the proverbial spinning skater's arms are pulled out because rotation is absolute even for GR even if the skater is far away from any star. Where as Mach conjectured that it is the presence of the rest of the universe (however far it may be) that confers some meaning to the notion of rotation and therefore produces the pull of their arms, it seems that this idea is now abandoned. I tend to think that Mach's principle came from the notion that empty space is nothingness. This is not the prevalent idea now. Space seems to be a bubbly thing after all, and filled with the Higgs field. Is this the thing against which rotation happens? What is the current thinking about this? Thank you.