The intergalactic medium is a sparse gas that connects galaxies, with a filamentous structure and an average density of 10 to 100 atoms per cubic meter. It is much hotter than the universe’s average temperature, with ionized hydrogen as its major constituent. The intracluster medium is a condensed area in the middle of galaxy clusters that radiates X-rays. The universe’s expansion will eventually tear everything apart.
The intergalactic medium (IGM) is a scattered gas that extends between the galaxies of the universe. It takes on a cosmic filamentous structure, with thin tufts and walls separating large empty 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 10 to 100 atoms per cubic meter. To put this into perspective: If you collected all the matter from a volume of intergalactic medium about the size of the Earth, you’d end up with matter equal to a small ball bearing.
Most of the universe is also quite cold: 2.73 Kelvin. This heat is leftover from the Big Bang. The intergalactic medium is also filled with a highly homogeneous radiation spectrum called the cosmic microwave background. Its existence has been used to support the Big Bang theory.
The intergalactic medium, the filamentary structure between galaxies, is much hotter than the average temperature of the universe: on the order of 100,000-10 million Kelvin. That’s because the gas heats up as it falls into the intergalactic medium from the huge voids that surround it. This temperature is sufficient to ionize or strip electrons from nuclei, so the major constituent of the intergalactic medium is ionized hydrogen. Physicists call this intergalactic medium hot-hot. In some areas near the intersections of the cosmic filaments, the temperature of the gas approaches 100 million Kelvin. While that sounds like a lot, it wouldn’t be very hot if you were in it, because the atoms that make it up are so scarce.
A particularly condensed area of the intergalactic medium is known as the intracluster medium, because it lies in the middle of galaxy clusters. The intracluster medium radiates X-rays which can be observed with a telescope.
Since the universe is expanding at an accelerated rate, the whole thing becomes sparser as time goes on. Eventually, perhaps in a few hundred billion years or perhaps longer, it will expand so rapidly that our galaxy and everything in it will be completely torn apart.
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