All lines of sight electrons in space are directly repelling each other with force F=Ke x e1e2/rr. That force is inherited, co-existed, therefore it transfers electrons kinetic energy instantly.
Sun light speed is infinite in space. Only when it pass through matter/medium/air/water it has limited speed, due to F=ma, all matters have mass, can never reach infinite speed.
Light speed in a medium is the rate of induction of electric force in that medium. For example, light speed in air is C, in water is 3/4 C.
Light must co-exist with matter. There is no light is space. There is no energy in space.