Question

When was the first black hole discovered?

Answer 1

The earliest known object widely considered to contain a black hole is Cygnus X-1, diacovered in 1964.

Explanation:

Cygnus X-1 is believed to have a black hole at its center because such a black hole would most naturally account for its obaerved X-ray emissions and its interaction with the gases from a companion star.

See here:
https://en.wikipedia.org/wiki/Cygnus_X-1