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Dark Matter is a kind of small material which is smaller than electron and photon. It is with no charge and could not be interfered by electrons. It can pass through the electromagnetic and gravitational field， which is an important composition part of the universe. In 1970， Dark Matter was discovered by Vera Rubin， researcher of geomagnetic department， Carnegie institute in Washington DC.

The density of Dark Matter is very small， but with a large number， so it has a huge mass. They represent 84.5% of the total material mass in the universe. Among them， human can only observe less than 10% （about 5%） kinds of materials in the universe. Dark Matter cannot be observed， but it can interfere with the light or gravity of the stars. We can obviously observe the existence of Dark Matter.

Through researches on gravitational lens， the formation of large scale structure in the universe， astronomical observation and theory of expansion of cosmology， modern astronomy reveals that the density of the universe is probably composed of 70% dark energy， 5% luminescence and not light objects， 5% hot dark matter and 20% cold dark matter.

Dark Matter is “dark” because the dark matter is considered only interacting with gravity， thus it makes dark matter invisible. But a group of international researchers， led by the researchers of Durham University declared that they probably discover the first potential evidence that the Dark Matter can interact between each other. This probably means that the Dark Matter is not “dark” at all. In history， it is considered that Dark Matter can be divided into three categories by movement velocity： Cold Dark Matter， Warm Dark Matter and Hot Dark Matter. The study of Dark Matter has become an important subject of modern cosmology and particle physics.

（Answered by Gao Shang- Editor of Knowledge is Power magazine）