Intergalactic medium
The intergalactic medium (IGM) refers to the diffuse gas and matter that fill the vast space between galaxies. Although invisible, extremely稀薄, and hot, it is far from empty: it contains a significant fraction of all ordinary matter in the universe and shapes the large-scale structure of the cosmos.
An Almost Invisible Gas
The intergalactic medium consists mainly of ionized hydrogen and helium (plasma), at temperatures ranging from a few thousand to several million kelvin. Its density is extraordinarily low—on the order of one atom per cubic meter in the most empty regions. This hot, diffuse gas is virtually transparent to visible light, making it very difficult to observe directly. It is primarily detected in X-rays and the far ultraviolet.
The Cosmic Web
The intergalactic medium is not distributed uniformly. It follows the large-scale structure of the universe, known as the cosmic web. Galaxies and galaxy clusters are located at the nodes of this web, connected to one another by filaments of hot gas. Between these filaments lie immense voids—the cosmic supervoids—virtually devoid of galaxies and gas.
The Missing Baryons
For a long time, astronomers could not find all of the ordinary matter predicted by the cosmological model. Recent observations have shown that a large portion of these missing baryons exists as hot gas in the filaments of the intergalactic medium— too tenuous and too hot to be easily detectable, but clearly present in the filamentary structures of the cosmic web.
Did you know?
The space between the Milky Way and Andromeda is not a complete vacuum. It contains a few atoms per cubic meter, extremely weak magnetic fields, and cosmic rays. This local intergalactic medium plays a role in the gravitational interaction between the two galaxies.
Frequently Asked Questions
Is the intergalactic medium really empty?
No. Despite its extraordinarily low density, it contains gas, photons, neutrinos, and dark matter. It is the closest thing to a vacuum that we can imagine, but it is not an absolute vacuum.
Can we travel through the intergalactic medium?
In theory, yes, but the distances involved are enormous. The distance between the Milky Way and Andromeda is 2.5 million light-years. At the speed of current space probes, this journey would take tens of billions of years—far beyond the age of the universe.
What is the temperature of the intergalactic medium?
It varies enormously depending on the region. In hot filaments, the gas can reach several million kelvin. In cosmic voids, it can be much colder. On average, the intergalactic medium is considered a hot, diffuse plasma.