Apparently only “dense” in comparison to other parts of space.
Raises issues for interstellar travel theories which entail sweeping in particles for fuel — but that is all speculative stuff anyway.
You must speaking of the Bussard Ramjet. I wish that was feasible but some people more knowledgeable than myself were saying not possible.
http://www.daviddarling.info/encyclopedia/I/interstellar_ramjet.html
http://en.wikipedia.org/wiki/Bussard_ramjet
As per Wikipedia ...
... 0.3 atoms per cubic centimeter; less dense than the average for the interstellar medium in the Milky Way (0.5 atoms/cm3), though six times denser than the gas in the Local Bubble (0.05 atoms/cm3) which surrounds the local cloud. In comparison, Earth's atmosphere at the edge of space has 12 billion atoms per cubic centimeter, dropping to 52 million at 150 km.
Soooooo ... yeah. ( Emphasis mine ... footnotes redacted. )