Dark matter is about 5 times as abundant as ordinary matter in the universe. We see its gravitational affects on large scales – within galaxies, in groups and clusters of galaxies, and in the overall way in which galaxies are spatially distributed on the largest scales.
Galaxies on the large scale are observed to be distributed in filaments and walls of excess galaxy density, along with voids of lower galaxy densities. These filaments and walls have been discovered during the past 30 years or so.
Since galaxies themselves are primarily composed of dark matter, it makes sense that the filaments and walls and voids are reflecting the large scale distribution of matter as well.
Basically, we can think of dark matter as the scaffolding upon which the ordinary matter clumps – the luminous matter seen from galaxies’ light emission and absorption and due to stars, gas, and dust within galaxies. And when we say light, we mean radiation at all frequencies including radio waves, infrared, visible, ultraviolet, X-rays and gamma rays. Radiation sources are due to ordinary matter, since dark matter couples very poorly to electromagnetic fields – that’s why it’s dark!
Map of our Neighborhood (you are in the center) http://www.atlasoftheuniverse.com/nearsc.html, Richard Powell, CC-BY-SA-2.5
So we know that filaments and walls that we see in the distribution of galaxies reflect (deliberate pun) the underlying distribution of the gravitationally dominant dark matter. It is the gravitational field of the dark matter which has controlled the clumping of ordinary matter into galaxies and clusters and superclusters of galaxies.
Here are some of the most important known filaments discovered over the past 3 decades:
- the Coma Filament
- the Perseus-Pegasus Filament
- the Ursa Major Filament
- the Lynx-Ursa Major Filament (LUM)
- the CIG J2143-4423 Filament
Now these are mostly named after constellations, but of course they are behind the constellations and external to our Milky Way galaxy, and at large distances. The most distant known appears to be at redshift z = 2.38, which means that the light comes from a time when the universe was less than 3 billion years old. It has a linear scale of 350 million light-years.
Galaxy filaments include also structures known as walls. And here are the major walls that have been discovered:
- the CfA2 Great Wall
- the Sloan Great Wall
- the Sculptor Wall
- the Grus Wall
- the Fornax Wall
- the Hercules-Corona Borealis Great Wall
The 3-D perspective map above shows superclusters and voids in our neighborhood. It is centered on the Milky Way and extends out to 500 million light-years. Superclusters are highlighted in blue, and 3 walls are highlighted in yellow: the Coma Wall, the Centaurus Wall, and the Sculptor Wall. A number of Void regions are highlighted in red.
The CfA2 Great Wall (also known as the Coma Wall) extends from the Hercules Supercluster to the Coma Supercluster to the Leo Supercluster on the right hand side of the map. It was the first wall discovered, in 1989, by Margaret Geller and John Huchra of the Havard-Smithsonian Center for Astrophysics (CfA).
This Hercules-Corona Borealis Great Wall is much further away, at a redshift of around z = 2, which means that we are seeing it from a time when the universe was a little over 3 billion years old (more than 10 billion years ago). It has an enormous length of around 10 billion light-years. Which is incredible, since the comoving distance to the Hercules-Corona Borealis Great Wall is 17 billion light-years.
Remember, when you look at these maps, you are also in effect seeing the distribution of the underlying dark matter.
In the next blog, we will talk about possible dark matter filaments on a much, much smaller scale, on the scale of the Sun and the planets within our Solar System, including Earth.