  This review presents the main points of current advances in the field of
active galactic nuclei (AGN). A brief historical excursion about the search for
the nature of AGN is given. The problem of close binary systems consisting of
supermassive black holes located in the centers of galaxies is discussed in
details. The main characteristics, as well as new methods for studying and
``weighing'' these new objects, are described. This paper is based on a
presentation made in the astrophysical seminar, which dedicated to the memory
of the outstanding astrophysicist N.G. Bochkarev (took place on May 19, 2023 at
the Sternberg Astronomical Institute of Moscow State University).
