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I'm may not be the best consultant on black holes but this is more about what constitutes sufficient evidence for a theory.
Are black holes just an illusion? Certainly their existence hasn't been proven without a shadow of a doubt; as witnessed by published lists that refer to these objects only as possible candidates for black holes.
The theoretical existence of black holes was predicted long before any candidates were found. The Schwarzschild radius describes the size of the sphere that a mass must collapse into and become a black hole. The surface of this sphere is popularly known as the event horizon. For example if all the Sun's mass were squashed into a sphere of radius 3km, it would become a black hole. General relativity provides models for the effects of black holes on matter and space but it does not prove the actual existence of such objects.
So what constitutes sufficient proof in science? The theory must explain observations and be consistent with all known facts. Continued observations must confirm the theory and verify any predictions made by it. This is the position with the theory of evolution. No new evidence has arisen to put it in doubt.
Where do we find some likely black holes (or BHs for short)?
BHs often form accretion disks and associated relativistic jets. These interactions cause strong X-ray and gamma ray emissions. The nuclei of quasars are believed to contain super-massive BHs that power accretion disks. There are more than 200000 known quasars.
Some strong candidates for super-massive black holes are Sgr A* in the Milky Way core and at the core of M87.
BHs may exist in binary systems. A BH might be seen to accrete matter from an orbiting star. This is the believed scenario for Cygnus X-1.
Gravitational lensing might turn up some evidence for BHs.
So what do we get out of this? Theoretical physics say BHs are possible. There is a huge number of candidates which appear to exhibit BH activity. New observations are producing better evidence for BH activity. It appears BHs are the most feasible explanation for the observed phenomena.
With better understanding, astronomers are developing more definite signatures for BHs e.g. event horizon dark shadow or evidence of frame-dragging.
Do they actually exist? My feeling is that the evidence is building to a strong "yes".
Regards, Rob
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