From the time we were first taught about outer space in elementary school, we learned that interstellar space was a vacuum. Basically, we could assume that all mass in the universe was concentrated at stars, planets, or other massive objects while the space between them was a perfectly empty void. While this is a good approximation, it is not entirely accurate.

Our text has made it clear that interstellar space is full of particles and energy, from photons to neutrinos to the interstellar medium to cosmic background radiation. If it did not have any of these things, space would be a perfect vacuum with no energy, stuck at absolute zero (0K). However, this is not the case. Even in the regions of space furthest from any heat sources, the cosmic background radiation keeps interstellar space at a temperature of about 3K. This only increases as we move toward stars, meaning there must be massive particles in space.

But how many? The most widely accepted figure for the density of interstellar space (M. Tadokoro, 1968) is calculated to be 7 x 10^-29 grams per cubic centimeter, which translates to about 40 hydrogen atoms per cubic meter. The best vacuum ever constructed on Earth was done at CERN at reported to achieve a density of about 1000 atoms per cubic centimeter. While this is astonishingly low, it is still over 2 million times more dense than interstellar space! No, space is not a perfect vacuum, but it is certainly the best approximation that exists.

Sources and Further Reading:

http://en.wikipedia.org/wiki/Vacuum#Outer_space

http://adsabs.harvard.edu/abs/1968PASJ…20..230T

http://www.ccmr.cornell.edu/education/ask/?quid=1026