A form of distributed intelligence; an intelligent being with parts spread across an entire galaxy. If not linked by wormholes, the internal communication lags would be on the order of tens of thousands years, making the top level very slow (but subminds could be much faster). The parts could be jupiter brains or other intelligent superobjects. While some Keterists and MPAists have suggested that the technosphere is evolving towards a state of a single Galaxy Brain, and ultimately a single Universe Brain, there is no evidence that this is so, as SI:<1 sentients are just as common, proportionally speaking, now as they were during the early Empires period.
It has been theorized that some, or many, distant galaxies have already been converted into galaxy brains by xenosphont civilisations. While little direct evidence of this conjecture is yet available, the Red Globular Galaxies, Leo Hyperobject 1, and the Leviathan are all cited as examples of potential galaxy brains. Critics of this position point out that while the existence of these constructs certainly demonstrates that galaxy scale engineering is possible, they provide no evidence one way or the other about whether they are the product of (or home to) a single galaxy spanning intellect or a host of lesser minds working together.
It may be the case that Galaxy Brains are not necessary, since computation using artificially constructed basement universes might offer a more efficient option.