The idea arose when astronomers found that the mass of large astronomical objects, figured out from their gravitational effects, was much greater than the mass figured out from the "luminous matter" they contain: 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, 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. 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.
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. 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.
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- Ade, P.A.R.; Aghanim, N.; Armitage-Caplan, C.; et al. (Planck Collaboration) (2013). "Planck 2013 results. I. Overview of products and scientific results – Table 9". Astronomy and Astrophysics 1303: 5062. http://www.sciops.esa.int/index.php?project=PLANCK&page=Planck_Published_Papers.
- Francis, Matthew (2013). "First Planck results: the Universe is still weird and interesting". Arstechnica.
- "Planck captures portrait of the young Universe, revealing earliest light". University of Cambridge. 2013. Retrieved 22 March 2013.
- Ferris, Timothy. 2015. Dark Matter. National Geographic. 
- Dark matter observed. http://home.slac.stanford.edu/pressreleases/2006/20060821.htm.