Dark matter

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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 the mass of large astronomical objects got from their gravitational effects was much greater than the mass calculated from the "luminous matter" they contain: stars, gas, and dust.

Dark matter was first proposed by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way. Fritz Zwicky in 1933 used it to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Later, many other observations have suggested the presence of dark matter in the universe. The rotational 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 reasons.

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][4] Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total content of the universe.[5][6]

Because dark matter does not seem to give off or reflect light, x-rays, or any other radiation, the instruments which can find normal matter (like hot gas, stars, planets, and us) are unable to 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 it is there is by how it affects things we can see by gravity.

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

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