What is Dark Matter?
Only 5% is visible in the universe
When we investigate the components of this universe, "visible" known matter such as protons or neutrons is only 5% of the universe. 5~6 times the quantity of ordinary matter is unknown matter (dark matter). The rest is unknown form of energy called dark energy（Fig.１）. We have observed the universe by the electromagnetic wave such as a visible ray, X-ray or infrared, however, "dark" matter cannot be observed by the electromagnetic wave. Then we call them "dark" matter.
A lot of evidences of the existence of Dark Matter
A various observation of universe have inferred the existence of dark matter. In the late of 1970s, from the measured the rotation curve of a spiral galaxy, the existence of the matter which is unseen, but has gravitational effects（Fig.２） was suggested. Another evidence is obtained by the gravitational lensing effect, which the very massive matter (large gravity) can bend light.
Dark Matter is the key of structure formation in the universe
In the recent studies, the universe has a large-scale structure which is a complex of galaxies, clusters of galaxies and voids. This structure has been formed as follows. The small density perturbations in the early universe（Fig. ３） made the gradient of the dark matter density. The more dark matter was concentrated in the dense parts by gravity, and gradually the "visible" dust and gas were also come together. As a result, stars and galaxies were formed. In this way, the dark matter is very closely related to the structure of the universe.
Success of the observation open the door to new physics and solution of mysteries of the universe
The dark matter is unknown matter, but these observation results infer some properties of the dark matter. (1) No electric charge, (2) very massive and (3) stable. The known particle cannot explain this kind of matter. Then we need unknown particles based on the new particle theories. The one of the candidates for dark matter is neutralino.
The dark matter is expected to exist one per 1 liter even in our neighborhood. However, we have not observed it directly. The direct detection of the dark matter is one of the most important issues in astrophysics. If we can succeed in detecting the dark matter, we can get a clue to understand what the dark matter is. When we clarify the mystery of the dark matter, the scenario of the universe will be revealed.