What is dark matter?

Dark matter is the matter that is invisible to light, making up the bulk of the mass in the universe. The components of dark matter is still unknown, which could be subatomic particles, black holes, or some other celestial bodies. Dark matter does not emit or absorb light on its own, but we can indirectly observe the existence of dark matter from the gravitational force acting on it. For example, the effect of gravity on the motion of galaxies and the light emitted from galaxies indicates that dark matter is not evenly distributed but is densely or sparsely distributed in the universe.  Spherical or spheroidal clumps of dark matter are called halos. It is likely that most galaxies in the universe reside within dark matter halos. Dark matter has also played an important role as a seed for structures such as stars and galaxies in the universe. It is believed that ordinary matter such as hydrogen and helium gas is attracted to the halos of dark matter, which then aggregated to form stars and galaxies. From the viewpoint of the structure formation of visible matter in the universe, dark matter can be roughly classified into the three types by the velocity dispersion at the time of its birth: cold dark matter, which was moving at much less than the speed of light; warm dark matter, which was moving at nearly the speed of light; and hot dark matter, which was moving at almost the speed of light. However, if dark matter moves too fast for a long time, the spatial distribution of hydrogen and helium gas becomes so smooth that it becomes difficult to form galaxies and clusters of galaxies. Therefore, "cold dark matter" or "warm dark matter" is now considered as the most likely candidate. 

In order to elucidate the nature of dark matter, the Cosmology Laboratory (Inoue Lab.) of the Department of Physics, Faculty of Science and Technology, Kindai University, is conducting observational and theoretical studies of gravitationally lensed objects. For recent results, please click here.