Intergalactic Medium

For those of you who want to go out beyond our galaxy, be forewarned. Intergalactic space is not empty. It's filled with hot 10 to the 7th power K, plasma. Fortunately, it's very sparse. It's mostly hydrogen that has been ionized. However, there is some neutral hydrogen.

How do astronomers know that this stuff is there?

There are two methods. One involves using an array of radio telescopes to pick up the radio blips from galaxies, probably from neutron stars or pulsars. These signals should be clean with mostly single frequency waves, but when these signals pass through the intergalactic medium, they are diffused and the signal is composed of different frequencies. Astronomers use this method to measure the density of the Intergalactic medium.
Another method is to use the Cosmic Web Imager, a detector that has been installed on the Hale 200-inch telescope at Palomar Observatory. This has enabled Caltech astronomers to detect the dim matter that exists between galaxies. Note that this is not dark matter. It's just matter that's very diffuse.

This Cosmic Web imager device has allowed the astronomers to detect dim matter in between a quasar-a supermassive black hole gone mad in a young galaxy-and Earth. In one case they could actually observe a million-light-year long filament flowing into the quasar, which could be helping the galaxy to grow. What helps in this observation is the fact that the Cosmic Web imager combines images taken at different frequencies to create a spectrographic image. This allows astronomers to identify the intergalactic medium's material, which in many cases is hydrogen.
The intergalactic medium can also be observed in the X-ray part of the spectrum. X-rays are the result of the plasma being heated to millions of degrees. This X-ray effect is most noted among galaxy clusters.

Photo-heating and adiabatic cooling from the expansion of the universe controls the temperature of the intergalactic medium. As the density of the medium decreases the temperature increases. This medium is also not uniform but is structured in a filament arrangement that indicates that gravity may play some role in dispersing it.

What all of this indicates is that there is much more matter in the universe than we thought. This is just another possible way that matter is dispersed throughout the universe.

Thanks for reading.