For questions about vacuums and partial vacuums or spaces largely devoid of matter.
A pure vacuum contains nothing. It is simply an empty space. Outer space is often cited as a vacuum, though gas and dust drift through many parts of it. In fact, "empty" space is not a true vacuum because of quantum effects: virtual particles constantly pop in and out of existence.
Don't use this tag merely because the question involves outer space (consider using the space tag).