To the best of my knowledge, Einstein didn't specify that objects with mass gravitate toward one another due to their being pulled. Einstein actually stated that space-time curves in the presence of mass, which then creates a geometry in space-time. Wheeler Explained it best, when he said... [ space-time tells matter how to move and matter tells space-time how to curve] Einstein even told us that gravitation is not a force, so why are supposedly rational people still searching for gravitons ???
There exists a form of energy which permeates the total volume of our universe, The energy is of unknown genre and is deemed to be responsible for the current phase of expansion of our universe. This as yet unknown form of energy bears the prefix dark energy, with the prefix "dark" denoting that no one can explain it.
To cut a long story short... if we have already accepted that there is a cosmological constant AKA lambda, why has Zwickys postulate regarding the existence of dark matter (and its accompanying gravitation) been accepted as theory? Gravitation would work equally well and the effect would be indistinguishable if it was caused by a push from the cosmological constant AKA dark energy, rather than a pull from (Dark Gravitons???)
There is absolutely no need for Dark matter, What's needed is a better understanding of gravitation.
This topic is one I have pondered long and hard and I have actually found a solution that could lead to a unified field theory, However the solution is so bizarre I'm still attempting to debunk it myself, (but to no avail.)
Is it possible that our reality could emerge from an effect caused by something considered to be a concept, (yet is also a cornerstone of our reality ?) When I say bizarre, its bizarre with a capital B.