If your talking ab0ut a substance that is largely photons you are talking about hydrogen or helium in outer space. If thier kinetic energy is high their electrons are stripped. They can make a substance with a very high specific gravity like in proton stars. In science fiction each episode is dependent on what is obviously impossible from today's point of view. For science to approach this pinnacle of technological advancement seems outlandish. Within Voyager's three dimensions of sub-space , attaining one billion miles per second is a matter of course. I can immagine the engines would have to be more powerful and bigger to serve a greater mass. To carry a high specific gravity they would need to be more expensive because of the greater warp factors surrounding the objects.