[noncount] : a quality that makes you continue trying to do or achieve something that is difficult. 1  What he lacked in talent he made up for in determination. 2  dogged/fierce/steely/stubborn determination.
The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958.
At best, Einstein thought black holes might be theoretically possible, but they weren't the sort of thing that would, or even could, ever be observed. Today, that era is long past. Black holes remained theoretical for some time, but they have become a standard feature of astrophysics, a practical reality.