Dark matter

Dark matter is invisible. The effect of gravitational lensing causes multiple images of the same galaxy. A ring of dark matter has been suggested to explain this.
In this image of galaxy cluster (CL0024+17) the dark matter is seen in blue.^[1]

Dark matter is a type of matter thought to be responsible for much of the mass in the universe.

The idea arose when astronomers found that the mass of large astronomical objects, and their gravitational effects, was much greater than the mass from the "luminous matter" that contains stars, gas, and dust.

Dark matter was first proposed by Jan Oort in 1932 as a reason for the spinning speeds of stars in the Milky Way. Fritz Zwicky in 1933 used dark matter to explain "missing mass" in the spinning speeds of galaxies in clusters. Later, many other observations have suggested that there is dark matter in the universe. The spinning speeds of galaxies,^[2] gravitational lensing of background objects, the temperature distribution of hot gas in galaxies and clusters of galaxies: these are some of the examples that make scientists believe in dark matter.

According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.^[3]Thus, dark matter is estimated to make up 84.5% of the total matter in the universe, while dark energy plus dark matter make up 95.1% of the total "stuff" in the universe.^[4]^[5]

Because dark matter does not seem to give off or reflect light, x-rays, or any other radiation, the instruments that are used to find normal matter (like hot gas, stars, planets, and us) can't find dark matter. It seems that dark matter is not made of the same thing as the matter we see every day on Earth. The only way we can tell if dark matter is there, is by how it affects things we can "see" by gravity.

In 2006, a group of scientists claimed that they had found a way to find dark matter.^[6]^[7] Since dark matter is supposedly very different from normal matter, it is expected to act differently. The scientists observed two far-away galaxy clusters that had crashed into each other at high speed: normal matter would have been scattered nearby after the collision, while dark matter would not. By measuring gravity, they were able to detect what looked like two clouds of dark matter, with a cloud of normal matter (hot gas) in between them.

Dark Matter Media

This artist's impression shows the expected distribution of dark matter in the Milky Way galaxy as a blue halo of material surrounding the galaxy.
Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the 'flat' appearance of the velocity curve out to a large radius.
Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter – enlarge the image to see the lensing arcs.
Models of rotating disc galaxies in the present day (left) and ten billion years ago (right). In the present-day galaxy, dark matter – shown in red – is more concentrated near the center and it rotates more rapidly (effect exaggerated).
Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree survey.
3-D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.
Fermi-LAT observations of dwarf galaxies provide new insights on dark matter.
Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.
Video about the potential gamma-ray detection of dark matter annihilation around supermassive black holes. (Duration 0:03:13, also see file description.)