Abstract: The Nobel Physics Prize in 2020 was awarded to three scientists for their fundamental contributions to the research of black holes and the discovery of a supermassive compact object located in the center of the Milky Way, providing clues to understand the fates of supermassive celestial objects in the universe. Nowadays, it is generally believed that supermassive black holes are located in galactic centers of all galaxies, and that they play key roles in galaxy structure formation and evolution. Those formed in earlier eons and provided energy sources in reionization of the universe, and shed light on our understanding of cosmological expansion history, dark energy physics, and nano-Hz gravitational waves of close binaries of supermassive black holes. The future joint observations of reverberation mapping campaigns and GRAVITY/VLTI of active galactic nuclei will help us make further progress in cosmology and gravitational wave studies.