General Astronomy/Black Holes in Hiding

Invisible Black holes
By definition, black holes do not emit light, so observing them becomes a bit problematic. However, the lack of emitted radiation directly from a black hole does not mean that we cannot detect its presence.

Gravity
First off, black holes have gravity, the remnant gravity left over from the mass that formed the black hole in the first place. The mass itself has gone into hiding—in fact we do not know what exactly has happened to it—but the gravity remains.

The gravity of a black hole is not stronger than the gravity of a star of the same mass, but because the black hole is so much smaller, the gravity at its 'surface,' sometimes called the "event horizon," is more concentrated and intense. It affects things near the black hole, and sometimes these effects can be detected by astronomers.

For example, if a black hole is part of a binary or multiple system, with another star (or stars) orbiting along with it, it may be possible to detect the presence of the black hole by the motion of the other objects, which astronomers can see.

As shown by Einstein's General Theory of Relativity nearly 100 years ago (1915), gravity can distort light. So if the light of some distant star (or more likely, a galaxy) passes very near the surface of a black hole as it beams through space to the Earth, it can be distorted in a predictable way. If astronomers detect these distortions, they can reasonably conclude that a black hole is somewhere along the line of sight.

Electromagnetic Energy (Light)
Even though a classical black hole cannot radiate electromagnetic energy, objects that have been caught by a black hole's gravity and are falling into it can emit radiation of characteristic types before they fall through the event horizon. Any falling or orbiting object is subject to gravitational accelerations, and any charged particle that is accelerated emits electromagnetic energy. In many cases, it is reasonable to expect that there are many subatomic charged particles (mostly electrons and protons) orbiting around or falling into black holes. When they do so, the give off electromagnetic energy with particular characteristics, which can be detected at Earth. In fact, the first candidate black hole, Cygnus X-1, was detected by just this technique, through the strong X-radiation coming from near the black hole. Black holes are much like messy eaters. Some material won't reach the event horizon but instead is caught up in powerful magnetic fields that exist around the black hole. These "jets" not only shoot some material away. They also emit prolific amounts of energy from radio waves to visible light to X-rays. The jets of material shooting from the central black hole of the Perseus cluster have blown out large holes (cavities) in the nearby gaseous medium.

Hawking Radiation
A classical black hole cannot emit radiation, but British astrophysicist Stephen Hawking predicted that through the non-classical processes of quantum mechanics, particles from a black hole could very slowly seep out into space and escape in a kind of quantum evaporation. This is called Hawking Radiation. In a very long period of time, black holes could actually dissipate through this process.

So far, no Hawking Radiation has been detected, although if it is, it could be another way of discovering and studying black holes.