You might imagine space to be empty. But the seeming void contains a barely perceptible fog of hydrogen and other neutral atoms. In 2020, NASA's New Horizons spacecraft, having passed the orbit of Pluto, measured a proxy for the amount of hydrogen around it. In the area between stars in our galaxy -- the "interstellar medium," or ISM -- New Horizons found around 130 hydrogen atoms per liter of space. Source
You might imagine space to be empty. But the seeming void contains a barely perceptible fog of hydrogen and other neutral atoms. In 2020, NASA's New Horizons spacecraft, having passed the orbit of Pluto, measured a proxy for the amount of hydrogen around it. In the area between stars in our galaxy -- the "interstellar medium," or ISM -- New Horizons found around 130 hydrogen atoms per liter of space.
Source