[semaphoreP]

It's almost all Hydrogen/Helium gas. Other larger atoms and molecules are known to exist in the interstellar medium, but it's primary gas.

Also, a good way to remember the density is ~1 Hydrogen atom per cm^3.

[ar15saveslives]

We could catch it and use as a fuel or recoil mass!

(does napkin math)

Nope :-(

[infogulch]

How about intergalactic densities?

[semaphoreP]

Don't know the number off the top of my head, but it should be considerably less. It should also be highly varying as some galaxies live in pretty densely packed environments (galaxy clusters), which some live in very open environments like our own galaxy.

[infogulch]

Nothing like a little "look it up yo damn self": Looks like it's about 1 hydrogen per m^3 [1] (via SO [2]), about 1 million times less dense. Damn.

  [1]: http://www.universetoday.com/30280/intergalactic-space/
  [2]: http://physics.stackexchange.com/questions/25378/how-vacuous-is-intergalactic-space/25379#25379