04-15-2015, 11:38 PM
One thing I think can be ruled out is galaxies with cultures that possess magical energy sources, such as the energy supposedly available in the Star Wars galaxy to run turbolasers, or the bilateral energy source used by the Culture to power their GSVs.
If a civilisation could literally pull energy out of another dimension like that, they would not need stars - they could just live on luxurious habitats in deep space (this is basically what a GSV is, after all). A civilisation with magical sources of energy of this kind could have unlimited growth until the luminosity of their magical energy exceeds the luminosity of the stars in that galaxy by an arbitrary amount.
If a civilisation could literally pull energy out of another dimension like that, they would not need stars - they could just live on luxurious habitats in deep space (this is basically what a GSV is, after all). A civilisation with magical sources of energy of this kind could have unlimited growth until the luminosity of their magical energy exceeds the luminosity of the stars in that galaxy by an arbitrary amount.