Galactic Rotation... no need for dark matter.

[Hetman]

What do you mean ("has never been rationally justified", "has not yet been justified rationally")?

Specifically, what does the process of 'rational justification' entail?

To put it bluntly: GR is non-local in nature, just like Newton's law.
Nothing new, just another representation of the acceleration field (with a few artifacts, which are the inevitable consequence of the method itself, namely the full geometrization of gravity).

The process of rational justification is a standard procedure.
And in this case it consists of:
localisation a sources of gravity (real, not only in the form of a name - quality), a mechanism for transferring energy - momentum, the discovery of various dependencies, circumstances, etc.