This document provides information about black holes, including their structure, formation theories, detection methods, and two specific black holes - Sagittarius A* and Cygnus X-1. It describes the key components of black holes as the singularity at the center with infinite density, the event horizon boundary, and accretion disks of spiraling material. Two main theories discussed are that black holes form from collapsed massive stars and act as tunnels to other universes. Sagittarius A* is located at the center of the Milky Way and Cygnus X-1 is a black hole candidate in the Cygnus galaxy.