Cubic centimeter, and it depends on where. For instance, in the Solar System, between stars, or between galaxies? In the Solar System, the solar wind has an average density of 3 to 10 particles (mostly protons and electrons) per cc, but it's lower the farther out from the Sun.
I think that is the estimate for the warm intergalactic medium; within galaxies, densities are more like 100-1,000 H atoms / meter at their most sparse.
So intergalactic space is “crowded” compared to, say, the vacuums at CERN, but quite “spacious” compared to the outer space between stars in a galaxy (and certainly compared to the space between the Earth and the moon).
I guess I’m not sure what an “appropriate” density would be :) 1 measly ice-cold hydrogen atom within a cubic meter is slim pickings.
>So intergalactic space is “crowded” compared to, say, the vacuums at CERN
I vaguely recall Sam's Laser FAQ claimed that terrestrial vacuum pumps couldn't do as well as outer space, due to outgassing of materials and diffusion through vacuum chamber walls.
The CERN page on the LHC vacuum system says it gets down to 10^-13 atmospheres. https://home.cern/science/engineering/vacuum-empty-interstel... The wikipedia article on vacuum gives a range for "deep space" from 10^-10 to 10^-20. Interstellar, but maybe not intergalactic.
I'd be curious about the source. Is that one He in the actual vacuum, or is that one He by dividing known matter by known space. If its the latter, that's very different.
It depends on where we are talking about. Inside the solar system is the heliosphere [1] with lots of particles from solar wind. As you leave the solar system you get the interstellar medium [2], which is about 90% hydrogen and 10% helium, most of it about half a particle per cm^3. Between the galaxies there's the intergalactic medium [3], which is speculated to contain about half of (normal, not-dark) matter in tendrils with a density of about 1-10 particles per cm^3 (and a lot less outside those tendrils).
The best vacuums we can produce artificially on earth have a couple billion particles per cm^3.
Perhaps not as different as you think! Most of the "known matter" is indeed H and He drifting in otherwise "empty space". Stars, while very dense, are few and far between on cosmic scales. In terms of mass, the ratio is about 4:1. So you'd get quite similar answers from both calculations.
Is that true? Seems kinda crowded out there.