A black hole is a region of space where gravity is so strong that nothing — not matter, not even light — can escape once it crosses a boundary called the event horizon. They're not "holes" in any literal sense; they're extraordinarily dense concentrations of mass, packed into an extremely small space.
How they form
Most black holes form when a massive star runs out of fuel and collapses under its own gravity at the end of its life. If the collapsing core is heavy enough, no known force can stop the collapse, and it crushes down into a black hole. These are called stellar-mass black holes, and they typically weigh a few times to a few dozen times the mass of our Sun.
There's also a much larger category: supermassive black holes, which sit at the centers of most large galaxies, including our own Milky Way, and weigh millions or even billions of times the mass of the Sun. How they grew so large is still an active area of research.
You can't see one directly — but you can see its shadow
Because no light can escape a black hole, you can't photograph one directly. What scientists have photographed is the glowing ring of superheated gas and dust swirling around a black hole's event horizon, silhouetted by the black hole's shadow. The Event Horizon Telescope project produced the first-ever image of this kind in 2019, showing the supermassive black hole at the center of the galaxy M87, and followed it in 2022 with an image of Sagittarius A*, the supermassive black hole at the center of our own Milky Way.
Time gets strange nearby
One of the stranger predicted effects of a black hole's gravity, confirmed by real observations, is that time itself runs slower the closer you get to it — a consequence of Einstein's general relativity. An observer watching a clock fall toward a black hole would see it tick slower and slower, appearing to freeze entirely right at the event horizon, though from the clock's own perspective, it would fall through completely normally.
Why people care
Black holes are one of the few places in the universe where the known laws of physics are pushed to their absolute limits, making them a natural laboratory for testing our best theories of gravity. They're also a genuine mystery at their core: what actually happens at the singularity, the theoretical point of infinite density believed to exist at a black hole's center, is something no current theory can fully describe.