Invisible mass
In other words, something different from the atomic matter (also known as baryonic matter) that we are familiar with. But what is it then? Nobody really knows! In fact it has never even been directly detected, because it doesn't emit visible light or other electro-magnetic radiation, the main way in which astronomers can observe the universe.
For this reason it has been named 'Dark Matter'.
So how do we know it actually exists? Mainly due to strange gravitational effects that have been observed, that must be be caused by something other than visible matter.
In 1933 Swiss astronomer Fritz Zwicky was the first to notice this, by observing the rate at which groups of galaxies rotate. A rotating object needs a certain amount of mass in order have enough gravitational attraction to hold together and not disintegrate, yet the amount of mass calculated from the visible stars was much less than it should have been. It seemed there was some invisible matter there making up most of the shortfall. And not a small amount either – about 80% of the matter in the universe appears to be invisible to our telescopes.
One dramatic gravitational effect of any sort of matter (visible or dark) is the way that a large enough concentration creates a gravitational field strong enough to distort space and thus to bend light, warping and magnifiying it. For example, the image above shows the light of a distant blue galaxy being distorted into a ring by the gravity of the nearer red galaxy. This ring shape is termed an 'Einstein Ring' (The exact shape of the distorted image depends on the relative alignment of the two objects – for example, multiple images or arcs may also be produced.) This effect also provides indirect evidence of existence of dark matter – by comparing the actual amount of distortion with the amount expected due to the visible matter, the amount of dark matter in the nearer galaxy can be calculated.
Gravitational lensing analysis has been developed so such a level that it is possible to produce maps of dark matter distribution in various parts of the sky, such as this one.
But so far all the evidence of dark matter has been indirect – we may know that it is there, but we have no clear indication about what it actually is made of. So the hunt is on to detect it directly. Various experiments with this aim have been carried out over recent years. To date none have provided conclusive results, but could the results from the Alpha Magnetic Spectrometer on the ISS finally be a definitive detection of actual Dark Matter? As yet it is too early to say but the researchers are confident that the mystery will be solved with furher experimentation. Watch this space!
http://news.discovery.com/space/dark-matter-mystery-deepens-140617.htm
https://newsoffice.mit.edu/2014/alpha-magnetic-spectrometer-detects-positrons-0918
http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
http://www.quantumdiaries.org/2013/06/26/does-dark-matter-really-exist/
1. NASA https://newsoffice.mit.edu/2014/alpha-magnetic-spectrometer-detects-positrons-0918
2. ESA/Hubble & NASA http://commons.wikimedia.org/wiki/File:A_Horseshoe_Einstein_Ring_from_Hubble.JPG
3. NASA, ESA, and D. Coe (NASA JPL/Caltech and STScI) http://www.nasa.gov/mission_pages/hubble/science/dark-matter-map.html