What is the Intergalactic Medium?
The intergalactic medium (IGM) is a sparse gas that extends between the galaxies of the universe. It takes on a cosmic filamentary structure, with thin wisps and walls separating vast void areas. The intergalactic medium connects galaxies together like a game of connect-the-dots. The intergalactic medium is quadrillions of times more sparse than so-called thin air. Scientists estimate the average density of the universe as a whole to be about one hydrogen atom per cubic meter, with the intergalactic medium having about 10 to 100 atoms per cubic meter. To put this into perspective: if you gathered all the matter from a volume of intergalactic medium about as large as the Earth, you would end up with matter equal to a small ball bearing. The majority of the universe is also quite cold: 2.73 Kelvin. This heat is residual from the Big Bang. The intergalactic medium is also filled with a highly homogeneous spectrum of radiation called the cosmic microwave background. Its existence was used
