Thread: Squaring Einstein's "relativity of inertia" with Mach's Principal and gravitational potential energy

How do we square Mach's Principal with Einstein's so called "relativity of inertia"? In other words, does anyone have any info on how masses (heavenly bodies) increase their inertia when they are moved closer together? Would this implicate so called "relativistic mass"? This notion seems contradictory to grav. potential energy, which would insist that energy decrease in this scenario. Yet Einstein himself said that "the inertia of a body must increase when masses are piled up in its neighborhood". Is this not essentially what Mach said too, that distant cosmological masses determine local physical laws? How can we relate these issues to the so called "Dark Matter" riddle?