Also referred to as black hole, an enormous concentration of mass the gravity of which is so strong that even light is not able to escape. The radius inside which this effect becomes evident is called event horizon. The existence of black holes has been confirmed by real world physics. There are different types and sizes of black holes, some of them actually existing, others assumed possible. Their common characteristic is the strong gravitational force that causes any kind of mass/energy to fall towards its center, once it is inside the event horizon. The mass concentration in the center is possibly beyond the current laws of physics. Even the elementary particles that form the atom nucleus under normal conditions might be crushed and further compressed to an unknown degree, with a density close to infinity, which is referred to as singularity.