This is a good example of how scientific knowledge works as 30 years ago Stephen Hawking speculated that there wasn’t a black hole in Cygnus X-1. Given an additional 3 decades of observation w/ increasingly better instruments, scientists now have enough additional information to be confident this is a black hole.
NASA article about this black hole.
http://www.nasa.gov/mission_pages/ch.../cygnusx1.html
Illustration of Cygnus X-1.