Scientists think that black holes are located throughout the universe, but we have only found several. The first bit of proof that a black hole existed was found in 1994, when the Hubble Space Telescope discovered something around galaxy M 87. Hubble discovered gasses that were speeding up towards a central point. The gasses showed that an object of 2.5 to 3.5 billion solar masses was present in galaxy M 87, which is now known to be a black hole.
Black hole in galaxy M87
In 1995, another black hole was discovered, this time it was found by array of radio telescopes, called the VLBA (Very Long Baseline Array). VLBA found a black hole in galaxy NGC 4258.