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Dark matter

One of the two greatest mysteries of the universe


Kavli Institute for the Physics and Mathematics of the Universe

Dark matter is an unknown substance, unevenly distributed throughout the universe and invisible to the human eye. We may not be able to see it, but its existence has been inferred from a multitude of indirect evidence, the first of which was the motion of galaxies noted in the 1930s. Unfortunately, we still have absolutely no idea what it is. We do however know what it isn’t. From observational data we know that dark matter is not any type of matter made from atoms, and that it isn’t any known type of celestial body or elementary particle. The remaining option, and the mainstream theory, is that it may be some form as yet undiscovered heavy elementary particle.

Video. Numerical simulation using 120 million particles. © Naoki Yoshida.
Gravitational attraction between particles of dark matter leads to uneven distribution and the formation of structure within an initially uniform space. If space was composed only of normal matter it would have no structure, matter would remain evenly distributed and stars and galaxies would never form.

We have also learnt that this almost totally unknown dark matter is a very important component of the universe. Exactly ten years ago in 2003, analysis of cosmic microwave background radiation revealed that dark matter accounts for some 80% of all matter in the universe. The atoms we know well are only a minor part, while this unknown dark matter comprises most of matter in the universe. In addition, we know that dark matter’s gravitational effect was required for the formation of stars and galaxies (video). If it weren’t for dark matter, there would be no galaxies, no stars, and no people. This is why Director Hitoshi Murayama of the Kavli Institute for the Physics and Mathematics of the Universe often describe dark matter as our “estranged mother.”

Researchers are doing their best to pin down the identity of dark matter in a variety of ways. Some are trying to create their own dark matter. It may be possible to create hereto unknown heavy particles in the Large Hadron Collider (LHC), which produces the highest energy collisions ever achieved in human history. Another approach is to try and detect the dark matter that exists in space. The XMASS detector built 1,000 m underground in the Kamioka mine in Gifu Prefecture is the world’s largest experiment for the direct detection of dark matter. It will be possible to say that we have found dark matter when the LHC finds a new candidate particle and XMASS captures a particle of about the same mass. It should then be possible to tease out the characteristics of dark matter by comparing the results from these two experiments.

Indirect research is also ongoing into the phenomena that should occur if dark matter exists. In fact, there was a recent announcement that received some attention. The Alpha Magnetic Spectrometer (AMS) located on the International Space Station to detect cosmic rays detected a shower of high energy positrons. The data has yet to be validated, but it is possible that these positrons were created by the reaction of particles of dark matter. In addition, the Subaru telescope in Hawaii is building up a map of the distribution of dark matter in the universe by closely examining its gravitational effect on the formation of galactic clusters and the motion of stars. This is made possible by the Hyper Suprime-Cam (HSC), a huge digital camera jointly developed by the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) and Japan’s National Astronomical Observatory (NAO), and which makes it possible to capture an image of a very large area of the sky in one shot. Further, the PrimeFocusSpectrograph (PFS), currently under development, will make it possible to examine the distance between stars and determine in detail how dark matter made up the framework of the universe, an important aspect of its evolution. The exact distribution of dark matter calculated from observational data will not only provide hints as to the nature of dark matter itself, but in combination with observational data from the XMASS and AMS experiments will allow even more accurate analysis of the nature of dark matter.

Methodology Create dark matter Capture dark matter Detect antimatter released through annihilation of dark matter Research the distribution of dark matter through observation of celestial bodies
Experimental facility (location) LHC (CERN, Geneva, Switzerland) XMASS (Gifu Prefecture, Japan), XENON (Gran Sasso, Italy), etc. AMS (International Space Station) Subaru Telescope HSC/PFS (Hawaii)

“So far, dark matter research has been a series ‘don’t knows,’ but recently there is a feeling in the air that soon we may discover something concrete. In the near future, there is great excitement at the prospect that we may even be able to catch a glimpse of our long-estranged mother.” says Director Murayama.

Interview/text: Azusa Minamizaki. Translation: Euan McKay.


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