You have two answers that are sort of correct but the real answer is we don't know. The physicists have calculated (based on the motion of galaxies which tells them some things about gravity) that roughly 80% of the matter in the universe is invisible to our best telescopes ("dark" matter). Of course, if the physicists are right about the speed of light being constant in space, then all the information they have about these other galaxies (and therefore, their calculations based on gravity, which is also assumed to be constant everywhere, is rather out of date (the time required for the light to get to earth from some of these galaxies is about on the order of the age of the universe). So... We really do not "know" anything about what is in interstellar or intergalactic space right now. On the other hand, we have no shortage of theories. So... Yes there will be electromagnetic energy zipping around (as waves or photons depending on whether the light is acting like waves or particles) and there will be the odd radioactive particle or atom. I've heard estimates of one atom per cubic mile in the "vacuum of space". There may be the even odder particle of "dust", a fragment of an asteroid or comet or planet or a star, and it would be even more lonesome, maybe one grain of dust is a volume that is one a.u. cubed. hope this helps