The 16-year study , now published in the astrophysical journal , has proved beyond doubt that lurking at the very centre of the galaxy is a black hole .
As a result , the team reports online this week inthe astrophysical journal letters , the earliest black holes grew less than 1 % over 200 million years .
Extrapolating from this small area , at least two million active black holes dot the sky , confirming that these extreme astrophysical objects are common indeed .