Q&A: Black Holes

Q:
Is it possible that a black hole could become so isolated that it becomes essentially invisible?

A:
Yes. There are three ways to detect black holes: (1) By observing X-rays or other forms of light emitted by gas falling into a black hole; (2) By observing the gravitational effects of black holes on nearby such as stars; (3) By observing the gravitational lensing of light by a black hole in front of a background star or galaxy. This last method is difficult to prove because other types of objects such as neutron stars are difficult to rule out. The first two methods, which have been used to discover many stellar mass black holes and supermassive black holes, require nearby gas or stars. If a black hole is so isolated that no gas or stars are in the vicinity, it will become essentially invisible.