Im trying to explain to this guy why time dilation occurs and how asking what is beyond the edge of the universe is irrelevant. Ill start with his original question. "Forgive me if I'm wrong and my memory has faded, but consider this: what is empty space? What is there between matter? What if the fact it was traveling much faster meant that it was encountering some kind of external friction, stronger than the stationary clock, which reduced the energy available to the particles oscillating in the atomic clock? My thoughts exactly are hard to put into words, hope that made sense." I then explained best I could but he refused to accept time dilation and is stuck on the idea that space has substance(aether). He then said: "So if space in itself is nothing but void, what is the difference between the void in deep space and the void beyond the boundaries of the known universe? It doesn't makes sense the three dimensions are present "just cuz"." Sorry for the huge amount of writing but Im not really sure where to go, I dont think i can really explain it adequately, I dont even have my degree yet and Im no teacher.