Einstein’s Relativity Theory emphasizes that "if a body radiates a given amount of Energy, that emitting body loses a Mass equal to that emitted Energy divided by the speed of light squared". But if that lost mass can’t be fully found by adding up all the resulting products, including negligible-mass high-energy Neutrinos; where did that mass go? My paper asserts that the lost (hidden) mass was ‘injected’ into the ‘aether’, increasing aether’s mass. As Einstein even said, in 1930, "Space is Eating-Up matter!" I use that "Einstein Statement" to estimate a minimum mass density of aether in Space, i.e., a key estimate but still likely much too low. And I also show that Neutrino propagation is likely an Ethereal Pulse or Stress, like a Twisting Spring Pulse (wave), instead of a forward or backward pulse. Thus, not likely a Particle mass flying through space, like a bullet or ‘baseball’. And I give more details, and address related questions.