A black hole isn’t actually a “hole” in space. It’s a region where gravity is so extreme that nothing can escape it - not even light. The boundary of that region is called the event horizon. Cross it going inward, and you’re never coming back out.
Inside the event horizon, the gravitational pull is so strong that the escape velocity - the speed something would need to move to break free - exceeds the speed of light. And nothing in our universe is allowed to go faster than light. So anything that crosses in, including photons of light, is trapped forever.
That’s why black holes look black. Light from the universe falls in and never bounces back. We don’t see the black hole itself; we see the silhouette of its event horizon against the bright stuff swirling around it. The first ever photo of a black hole, taken in 2019 of one in galaxy M87, shows exactly that - a dark circle (the event horizon) surrounded by a bright orange ring of glowing matter falling in.
