A black hole is a region in space where gravity is so strong that nothing, not even light, can escape from it. This happens because a massive object has collapsed in on itself, creating an incredibly dense point, known as a singularity, at its center. The boundary surrounding the black hole is called the event horizon—the point beyond which nothing can return once it crosses.

Black holes are formed when massive stars exhaust their fuel and collapse under their own gravity at the end of their life cycles. There are different types of black holes, including:

1. Stellar black holes: Formed from the remnants of massive stars after they go supernova.
2. Supermassive black holes: Found at the centers of galaxies, including our Milky Way. These can be millions or even billions of times the mass of the Sun.
3. Intermediate black holes: Thought to be in between stellar and supermassive black holes in terms of size.
4. Primordial black holes: Hypothetical black holes that could have formed in the early universe.

Although black holes cannot be directly observed, their presence is inferred by studying the effects they have on nearby matter, such as the bending of light or the behavior of stars orbiting around them.