Why Is Space Not Completely Empty Between Galaxies?

💡 The Answer

• The interstellar and intergalactic medium is filled with an extremely diffuse gas.
• This gas consists mainly of hydrogen and helium atoms leftover from the Big Bang.
• The average density is astonishingly low, estimated at roughly one atom per cubic meter.
• For comparison, Earth's atmosphere at sea level has about 10^25 molecules per cubic meter.