# How many atoms per light year does light encounter when traversing interstellar space?

Interstellar space is pretty empty but there a small number of of atoms (mostly hydrogen?) floating around. How many atoms per light year would a photon encounter while traversing interstellar space?

• @Raskolnikov Not my field, but I seem to recall numbers like $1/m^3$ for "local" intergalactic space and much less for the great voids. – dmckee --- ex-moderator kitten Nov 19 '10 at 3:56