One of the Voyager probes measured the density of the interstellar vacuum at 80,000 protons (and the same number of electrons) per cubic meter. A proton going through a piece of aluminum foil delivers a roughly constant amount of energy regardless of speed; a relativistic proton will pinch through and carry most of its energy with it.

(No punchline; I just think that's cool. I understand that the real problem is the rare dust grain, not the ubiquitous gas.)