General relativity (GR) passed many astronomical tests but in majority of
them GR predictions have been tested in a weak gravitational field
approximation. Around 50 years ago a shadow has been introduced by J. Bardeen
as a purely theoretical concept but due to an enormous progress in
observational and computational facilities this theoretical prediction has been
confirmed and the most solid argument for an existence of supermassive black
holes in Sgr A* and M87* has been obtained.Comment: 12 pages, 1 figure, presented as a talk at The 6th International
Conference on Particle Physics and Astrophysics (ICPPA-2022), accepted in
IJMP