Rawls makes clear in TJ that, following Hume, he assumes as one of the background conditions of his project the presence of "the circumstances of justice," that is, the objective and subjective circumstances "under which human cooperation is both possible and necessary" (p.126). The key conditions are that individuals "have different ends and purposes" that lead them "to make conflicting claims on the natural and social resources available," which resources are assumed to be moderately scarce (p.127). I think Rawls sees these conditions as having characterized most (if not all) societies, including the relatively affluent Western societies of the mid-twentieth century.
It's this emphasis on the Humean "circumstances of justice" that underlies Rawls's position that his theory is "a theory of human justice" (p.257, italics added). The theory does not apply to non-human entities or non-human societies that may not be subject to the constraints imposed by the circumstances of justice. Rawls writes (p.257):
...I have assumed all along that the parties [in the original position] know that they are subject to the conditions of human life. Being in the circumstances of justice, they are situated in the world with other men who likewise face limitations of moderate scarcity and competing claims. Human freedom is to be regulated by principles chosen in the light of these natural restrictions. Thus justice as fairness is a theory of human justice and among its premises are the elementary facts about persons and their place in nature. The freedom of pure intelligences not subject to these constraints, and the freedom of God, are outside the scope of the theory.And presumably for much the same reasons, galaxy clusters are also outside the scope of Rawls's theory.