Black holes are astrophysical objects of interest due to their immense gravitational attraction. A black hole occurs when more than a certain amount of matter and/or energy is located within a small enough space. Given a large enough mass in a small enough space, the gravitational forces become large enough that within a nearby region of space, nothing - not even light - can escape from inside that region to the wider universe. The boundary of that region is known as the event horizon because an observer outside it cannot observe, become aware of, or be affected by events within the event horizon. This region is the black hole's boundary, in effect.