What matter does the vacuum in the universe consist of?

Answer

Dear David,

You said it yourself: the airless space in the universe is airless. There is no air: no oxygen gas, no nitrogen gas, no CO₂, not ozone, nor any other substances that occur in the Earth’s atmosphere. There are no molecules or atoms, only empty space (vacuum).

In addition to celestial bodies, there are also dust clouds and gas clouds in the universe that sometimes consist of very exotic substances and sometimes of very well-known substances (e.g. small organic molecules), but as far as I know they only occur inside galaxies and not outside them. Between the galaxies are unimaginable amounts of truly empty space.