Black Holes, are portals which thrust you to a point many light years closer to the center of the galaxy. They are rare and exceptionally helpful for reaching the endgame. There are 3 known ways of finding a Black Hole. And the first of which involves speaking to a friendly Korvax named Nada.
When exploring the galaxy you may stumble across a "Space Anomaly" and inside you'll find two mysterious aliens who are able to speak in the travelers own language, named Nada and Polo. Speaking to Nada will net you the option to reveal a nearby Black Hole.
The second method of Black Hole discovery is through completing the main questline of the game, and completing the Atlas path. I won't divulge what happens at the end of the game here, but I will say that the next time you play, you will automatically be notified of local Black Holes.
The last method is to simply stumble into them, no skill involved, though a good deal of luck is necessary. Here's a video so you can see them in action.