It is not to be mistaken for a vacuum. There are hydrogen atoms, some helium atoms, a faint smoke of metals drifting away from exploded stars. Hot in a sense that does not register to humans, because it is so diffuse. A liter of the air in our biomes would have to be cast across hundreds of light-years to get it as diffuse as the interstellar medium. The whole voyage to Tau Ceti and back takes place inside the Local Interstellar Cloud and the G Cloud, which are concentrations of gas within the Local Bubble, which is an area of the Milky Way galaxy with fewer atoms in it than the galaxy has on average. Turbulence, diffusion: in fact, with our magnetic shield coning ahead of the ship, electrostatically pushing aside the occasional grains of dust big enough to harm it in a collision, all atoms of any kind encountered en route are pushed aside, so we register our surroundings mostly as a kind of ghostly impact and then as a wake, shooting by to the sides and then astern of us. It appears to vary between .3 atoms per cubic centimeter and .5 atoms per cubic centimeter.
