Approximately 170 billion (1.7 × 1011) to 200 billion (2.0 × 1011) galaxies exist in the observable universe. Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas with an average density less than one atom per cubic meter.