At the end of the 1960s and early 1970s, Donald Lynden-Bell and Martin Rees proposed that the Milky Way and perhaps most other galactic nuclei might contain a central massive black hole. But the evidence for such an object was lacking at the time because the centre of the Milky Way is obscured by interstellar dust, and was detected only as a relatively faint radio source.
In the late 1970s and early 1980s, Charles Townes and his collaborators including Reinhard Genzel, developed instruments capable of observing the centre of the Milky Way at infrared wavelengths, which can pass through the interstellar dust clouds with relatively little obscuration. By analyzing the spectrum of such radiation, they inferred that gas is swirling around a central concentration containing a few million solar masses. These authors suggested that the central object might be a supermassive black hole, but the observations did not have sufficient angular resolution to prove that conjecture.