>without messing up the equations for gravity too much. If some refractory effect (TBD) causes gravity to bend toward mass concentrations, then this effectively "confines" gravity into a plane. Then the propagation law becomes 1/r rather than 1/r^2.
There are objects like globular clusters and dwarf galaxies that lie outside the galactic plane, but are not that far away. If our galaxy's gravity was confined to its plane, you would expect nearby external objects to move differently from how they are observed to move. See illustration:
I was hoping someone would mention globular clusters! I couldn't find a way to integrate it naturally in my post.
I don't see any information about globular cluster movement in that image. Naturally the confinement wouldn't be 100%, and some amount of gravity would still "leak out,"[1] so the mere existence of globular clusters isn't proof one way or another.
Surveying globular clusters offers a good test. RG might (or might not) explain why globular clusters are mysteriously 1) preferentially orbiting perpendicular to the primary galaxy's rotation, perhaps due to stability, and 2) preferentially aligned with each-other in their own plane, perhaps due to mutual (lensed) gravitation. Both of these observations are otherwise unexplained (or require numerous free parameters) under dark matter.
We won't know until someone does a survey, and probably some simulation work. Sadly MOND soaks up most of the oxygen in the Modified Gravity field, leaving almost none for RG.
There are objects like globular clusters and dwarf galaxies that lie outside the galactic plane, but are not that far away. If our galaxy's gravity was confined to its plane, you would expect nearby external objects to move differently from how they are observed to move. See illustration:
https://cdn.britannica.com/23/4423-050-964A0A08/Distribution...