I came upon this question while watching a pop-sci video on youtube about Dark Matter and thinking about all the things that could be contributing gravitational influence to a galaxy.
From relativity we know that mass and energy is more or less the same and and both bend spacetime (i.e. cause gravity). And given how much energy stars give off and how big galaxies are, there is a lot of light, a lot of photons whizzing around and altogether that adds up to a sizeable chunk of energy. Relatively speaking maybe negligible next to ordinary or dark matter, but should be a big number. Some bounded volume would have to be defined, but I have no idea if physicists have definition for the boundary of a galaxy or what it is.