The identified potential black holes were observed with the NuSTAR X-ray space observatory to look for radiation from the hottest gas near the black hole.
Notes on verification
Multiple independent, high-quality sources (NASA, JPL, Caltech NuSTAR site) corroborate the claim verbatim, along with independent science media coverage.
Sources
- A third of black holes in the Universe could be hiding from view | BBC Sky at Night Magazine (skyatnightmagazine.com)
- https://www.jpl.nasa.gov/news/how-many-black-holes-are-hiding-nasa-study-homes-in-on-answer/ (jpl.nasa.gov)
- https://nustar.caltech.edu/news/nustar250113 (nustar.caltech.edu)
- https://www.nasa.gov/missions/nustar/how-many-black-holes-are-hiding-nasa-study-homes-in-on-answer/ (nasa.gov)
- https://scitechdaily.com/nasa-unveils-a-hidden-universe-of-supermassive-black-holes/ (scitechdaily.com)
- https://www.space.com/the-universe/black-holes/astronomers-find-hundreds-of-hidden-black-holes-and-there-may-be-billions-or-even-trillions-more (space.com)