I'm sorry, but I've been reading some online bullitin boards for a sci-fi video game, and I'd just like to make an announcement.

Space is empty.

For all of you who watch the intro to Star Trek Voyager and go "No it isn't! Look at all this stuff out there! Gas, dust, planets! Look at it all!", I say, "Well within anyone's right to say so, there is absolutely, satistically nothing in space (by volume). Literally, nothing." Also, "Why are you watching such a bad show? Except Teri Ryan, rawr."

If you take all the volume of "stuff" in a galaxy, and average it out across the whole galaxy, you end up with 1 particle of hydrogen occupying each cubic kilometer.

That's a lot of elbow room.

Average it out for the visable universe (ie. out to about red shift 7... measuring red shifts are actually the best scale for cosmological distance scales) and you end up with an "atmosphere" a hundred billion times more rarified.