This is one of the questions on the frontiers of our knowledge.
I think it's no wonder that we have difficulties reconciling quantum mechanics with relativity - look at the length of that spectrum. (Note that here we deliberately avoid the complexities of life, nature, and civilisation in the equation. Surely, they're not relevant. If need be we just call the planet that they all live on a lump of rock and move on).
The spectrum we're looking at goes from the quantum world, through molecules, rocky or gaseous structures, before finally we get to the planetary or stellar scale and this property makes itself felt through the curvature that its mass creates in space-time. It's an emergent phenomena (the curvature of space time) on a very big scale, which is *ages* away from the quantum scale which is where we’re trying to look for it.
I'm sure it's there, but no wonder it's hard to find - whatever causes it must be *bloody tiny*.
Perhaps the same issue extends to dark matter, and dark energy. Both are some seriously large scale phenomena (perhaps they are even additional forces? We know so little about them: who knows?), that you have to get to the level of *galaxies* to properly see them in action - and we're trying to find its source at the quantum level. That will take some incredible equipment.