The Universe in its largest dimensions remains a mystery because we do not yet understand the nature of most of its constituents: dark matter and dark energy. If the hypothesis of dark matter was made in the 1930, it is only in the beginning of this century that one has realized that the expansion of the Universe has been accelerating recently, a discovery which led to the Nobel Prize in 2011. This fact can only be accounted for by a significant change, introducing a dominant component, dark energy, in the Universe or modifying the theory of gravity on cosmological scales.

 A strong experimental program has been built in the recent years to study the nature of this phenomenon using different and complementary probes. Kavli IPMU of University of Tokyo has been leading one of the most important surveys aimed at exploring the nature of dark energy, the Subaru Measurements of Images and the Redshifts SuMIRe project relying on complementary observations carried out with the Hyper Suprime-Cam and soon, the Prime Focus Spectrograph, both on the 8.2-m Subaru Telescope. Both instruments are powerful and competitive with other survey instruments and facilities, and will play a significant role in cosmology and survey astronomy in the coming decade.

CNRS/IN2P3 and University of Tokyo teams have been collaborating on the design of a two-season ultra-deep supernova survey, targeting the 0.8<z<1.5 redshift range. This observing program has been completed in early 2020, and has delivered an exquisite dataset currently under analysis. This project provides a springboard towards the data analysis and exploitation of the Vera Rubin Observatory, as well as the European mission Euclid. Institute of Astronomy (IoA) of  University of Tokyo has operated a wide-field Schmidt telescope in Nagano. The Tomo-e Gozen camera on the Kiso Schmidt telescope, which is the first wide-field CMOS camera over the world, is finding new supernovae from very early phase, and origins of supernovae are being studied. IoA also leads TAO project , and the first light of the 6.5-m optical-infrared TAO telescope is expected in 2022 at the world highest site in Atacama, Chile.

Japanese researchers from IoA, RESCEU and ICRR, and French institutes, including IAP and CEA-Saclay, are also promoting observational research on massive galaxies and galaxy clusters, which are another probe of the dark side of the Universe via gravitational lensing or by using submillimeter-wave facilities including ALMA.