Give this a read if you’re curious!

https://www.sciencedaily.com/releases/2022/11/221104113504.htm#:~:text=hole%20to%20earth-,Gemini%20North%20telescope%20on%20Hawai'i%20reveals%20first%20dormant%2C%20stellar,hole%20in%20our%20cosmic%20backyard&text=Summary%3A,hole%20in%20the%20Milky%20Way.

Essentially, you have to be looking at the right place at the right time. We cannot detect black holes without the aid of their affects on physics around them. This one was trickier as it is dormant and therefore pretty impossible to detect. It lies in a binary system (sharing orbit with another star) so they witnessed this other star’s orbit get thrown off from a much denser object- this dormant black hole