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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.

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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.

1: https://en.wikipedia.org/wiki/Heliosphere

2: https://en.wikipedia.org/wiki/Interstellar_medium

3: https://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic...

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.

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