X-ray spectroscopy technology is utilized to understand the structure and formation of the universe and three-phases of researches are carried out simultaneously: technology development for future, development of space missions, data analysis of past missions. For future observations, transition edge sensor (TES) microcalorimeters and related technologies are being developed. The Soft X-ray Spectrometer (SXS) onboard the Hitomi (ASTRO-H) is the most recent space project for X-ray spectroscopy. To fully utilize the high-energy resolution of the microcalroieter array of the SXS, calibrations of the instrument are very important, and the ground and in-orbit calibration systems have been carefully designed. At the time of the Hitomi mishap, the SXS was still in the early phase of in-orbit calibration and the X-ray path was blocked by the gate valve. In such limited conditions, the SXS could resolve the fine structure of the K emission from highly-ionised helium-like iron (Fe) ions from the Perseus cluster, which is a major achievement for x-ray astronomy. The soft x-ray diffused background data obtained by the Suzaku observatoary over the course of more than a decade were used to search for signatures of dark matter, e.g. sterile neutrinos. The Earth’s magnetic field was utilized to impose an upper limit on axions produced by decays of massive particles.