The concept of a "monster black hole" is used to describe extremely large and rapidly growing black holes that were thought to have existed in the early stages of the universe, during a period known as the "quasar era." These black holes are believed to have played a key role in the formation and evolution of galaxies and the universe itself.

However, the term "monster black hole" is not strictly defined, and its usage may vary among scientists and researchers. Typically, it is used to refer to black holes that have masses exceeding billions or even trillions of times the mass of the Sun. Such supermassive black holes are known to exist at the centers of many galaxies, including our own Milky Way.

The existence of monster black holes is inferred from various observational and theoretical studies, including observations of massive galaxy clusters and jets of hot gas spewed out from the vicinity of supermassive black holes. However, direct detection or imaging of these monsters is extremely difficult due to their immense distance from Earth and the limitations of current technology.

In recent years, there have been several significant discoveries of supermassive black holes, pushing the limits of what was previously known and challenging our understanding of black hole formation and growth. As observational capabilities improve and new data is acquired, our understanding of monster black holes and their role in the universe continues to evolve.