Much of the mass in the universe lies not in stars or galaxies, but in the space between them, known as the intergalactic medium. It is warm and even hot, and is called the "warm-hot intergalactic medium," or WHIM. It holds about 50% of the normal mass (viz. baryonic, not including dark matter) of the universe but with a density of hydrogen ions less than 100 per cubic meter.
