We’ve heard one with gravitational waves and we’ve seen another because of how it interacts with other stars. You see, the main problem with looking for black holes is that we can’t see them! So we have to look at how they interact with the universe to measure their properties.
Lets start with the biggest one. NGC 1277. It weighs 17 billion times the mass of our Sun! It’s a super massive black hole and is centre of a pretty big galaxy. We use features such as how quickly other stars orbit in the galaxy. This black hole is so big astronomers didn’t believe it was real for a whole year! They kept redoing calculations until they were convinced.
The second one is slightly smaller, but it was the first Gravitational Wave signal we ever found. This black hole is actually made up of two smaller black holes that collided together. This black hole weighs approximately 100 times our sun. This was an important discovery for lots of reasons. It proved Einstein’s General Relativity was correct, it also showed us there exists a class of Black hole between the small and the really massive.