Cygnus X-1 is the most likely black hole candidate. First discovered in 1973, the black hole's companion blue supergiant star (HDE 226868) was detected orbiting around an unseen body. In addition, Cygnus X-1 is a strong source of x-rays. Centaurus X-3, identified around the same time, is also a likely black hole candidate (Miller).

A microquasar known as GRS 1915+105 has been emitting two oppositely directed jets of radio-transmitting gas. GRS 1915+105 is a suspected black hole orbited by an ordinary star. The directed jets appear to be travelling at superluminal speeds due to its direction, right at Earth (Cowen Tracking 357).

Mounting evidence also points to all quasars harboring black holes inside them.

Two galaxies, NGC 4258 and M87, are both suspected of harboring black holes. Astronomers were able to come to this conclusion by measuring the velocities of gas and dust around unseen objects. Judging by the data received, only a black hole could produce enough gravity to explain the enormous speed of the gases (Miller).

Today, astronomers believe that black holes reside in the centers of all galaxies, even in our own galaxy. According to Cowen in "Firm Evidence of Milky Way Black Hole," there are two main pieces of evidence used to support this idea:

The first piece of evidence was discovered in 1974. The discovery of a compact radio source, coined Sagittarius A*, was releasing emissions that are believed to be the last wisps of gas falling into this suspected black hole.

The second piece of evidence that supports this idea is that stars within 1.5 light-years of our galaxy's center "whip around at a furious rate." Astronomers believe that the orbiting stars are caught in the grasp of an unseen body (most likely a black hole).