It is a radio source (RS) discovered in 1974 within an area of the sky
already noted for its radio signal from the beginning of
radio astronomy. It was theorized to be a black hole for
some years, followed by considerable confirmation in 2002 with the
observation of orbits of stars in its vicinity (e.g., S2),
orbiting something unseen that clearly had the mass of an SMBH.
The Event Horizon Telescope (EHT), which was developed to resolve its event horizon
using millimeter astronomy, released its first image in 2022
after years of effort.