Since we can observe gravitational lensing and conclude that mass can affect the path of EM radiation it seems logical to me to assume that EM radiation will exert a slight gravitational attraction of it's own on a mass,- although I do not recall ever reading about this. Presumably the gravitational attraction that a single photon of EM radiation will exert is proportional to it's mass where m=E/c(squared) and the distance between the photon and the mass. Clearly we are considering an incredibly weak force if this is to be calculated. However, if the total gravitational attraction due to the total EM radiation within a galaxy is calculated this would now become significant. Could this explain the observed motion of the outermost objects within galaxies and therefore be a candidate for dark matter? Many thanks for reading.