Black holes are regions of space where gravity is so strong that nothing, not even light, can escape. They form when large stars collapse at the end of their life cycles. There are two main types - stellar black holes, which are relatively small and form from collapsed stars, and supermassive black holes, which are millions to billions of times the mass of our sun and reside at the center of most galaxies. Although black holes themselves are invisible, astronomers can detect them through their effects on nearby stars and gas.