In astronomy, what is a void?

Related questions

• Why do astronomers use scientific notation to describe sizes?
• What is astronomy?
• What types of jobs are available for those who hold a degree in astronomy?
• What is the meaning of the word astronomy?
• How would you calculate the energy of a photon with a known frequency?
• What is the difference between astronomy and astrology?
• What schools are renowned for their astronomy programs?
• What is a meteor?
• What occupations are there in the astronomy field?
• How is the acceleration of gravity calculated for planets?