Image from NASA
A black hole is an area in space in which the gravitational pull is so strong that nothing, not even light, can escape it. Because light can not escape, black holes are invisible.
The gravitational pull is so strong because the center of the black hole contains a very large amount of mass contained in a small space.
Black holes come in a variety of sizes. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Another kind of black hole is called "stellar." Its mass can be upwards of 20 times the mass of the sun. The largest of black holes are called "supermassive." These black holes can have masses that are more than 1,000,000 times the sun's mass. The supermassive black hole at the center of the milky way is called Sagittarius.
A black holes are invisible because gravity drags all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.
When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.