Stress-energy tensor and active mass
You are likely at the level where "we just don't know" comes to the fore.
I still find it incredible that without knowing the finest level of details, what is "really" going on, physicsts are able to concoct mathematics that predicts so many phenomena and confirm many with experimental evidence. The related part that's so crazy is the nature appears to follow mathematical constructs that we put together....
"Pressure is the flux of momentum",,,but who thought of that?? How many other thoughts were proposed, discarded and how did the "flux" interpretation win out???
In another thread here someone posted the components of GR tensors don't have physical interpretations but I fid that hard to accept...