Black holes in the real universe and their prospects as probes of   relativistic gravity
  Collapsed objects have definitely been observed: some are stellar-mass objects, the endpoint of massive stars; others, millions of times more massive, have been discovered in the cores of most galaxies. Their formation poses some still-unanswered questions. But for relativists the key question is whether observations can probe the metric in the strong-field domain, and test whether it indeed agrees with the Kerr geometry predicted by general relativity. 
Martin J. Rees