A black hole occurs when a large star, much larger than our sun, runs out of fuel.
The remaining mass in the star collapses in on itself, causing a vacuum and sucking everything around itincluding lightinto the “hole.” That said, scientists have never actually seen a black hole. They are what’s called dark matterthey aren’t visible.
So how do we know they exist? From the effects they have on visible matter around them. For instance, sometimes a large supply of magnesium, silicon, oxygen, and sulfur is detected on and around a living star.
None of these gases are produced by visible stars in large amounts; they are, however, produced in large quantities when a dying star like a supernova is burning much, much hotter. Black holes often pull at neighboring stars, too, causing them to wobble.
Scientists also know that when black holes suck in parts of a nearby star, X rays are emitted, so when big surges of X rays are detected around living stars particularly when one or more of these other indicators is present, it’s a fairly clear sign that a black hole is present.