Supermassive black holes and the dense stellar world surrounding them are one of the most extreme places in the universe. Black holes are a fascinating system: they are black and nothing, not even light, can escape the boundary of their edges. People often ask themselves: if gravity is so strong that light cannot escape from a black hole, how can we see it? A photon that narrowly circumvents this boundary can make a few revolutions around the black hole, but it is black. Everything that crashes into it stays in it forever, and no light escapes it. Black holes exist because they are part of the last great crisis: any matter can fall into a black hole, and if it does, it will never exist again, whether as a result of a supernova, an explosion of neutron stars, or some other catastrophic event. Black holes in the universe can be divided into two categories: medium-sized and supermassive black holes, both of which can also be seen in other parts of the universe, such as the Milky Way. The largest observation-based black hole in our universe and the largest in terms of mass is a supermassive black hole that extends from hundreds to billions of solar masses. Medium-sized black holes are about the size of our Sun, about 1,000 times larger than our Sun, and have a mass of less than 1 solar mass. Astrophysics suggests that the mass of the black hole is concentrated in a very small space - time roughly the size of the Milky Way. This mass creates such a strong gravitational field that nothing, not even light, can escape it. The low limit for the size of black holes is the theory that predicts and explains most of their features. The shadow of a black hole is the boundary that marks its boundary, where the escape speed is equal to the speed of light. The event horizon of black holes is such that light cannot escape from this boundary or mouth. As soon as everything that falls into a black hole passes the event horizon, it is drawn in and cannot leave it. Light and matter can move fast enough to escape the unstoppable gravitational pull of a black hole, but only at the speed of light. The cosmic trapdoor through which light and matter can escape, and it is invisible to us. As Stephen Hawking put it: "Anyone who falls into a black hole is torn apart by intense gravity and adds mass to it. The time it would take for a typical black hole to disappear at some point is very long, but the process by which it loses mass is also very slow for massive black holes - made of stars. It would take about 10,000 years, or about 1.5 billion years after the formation of the universe, for the black hole to evaporate. In contrast, general relativity, which explains gravity as coming from the curvature of space and predicts the existence of black holes, allows for an inverse process that brings back something that has fallen into a black hole. The question of whether or not there is a single black hole remains unresolved, although string theory offers a way. String theory could transform what is considered a "black hole" into a dense star - like an object that essentially amounts to a confused ball in space - time, like a string. Alternatively, a supermassive black hole could come from a giant star that eventually forms a black hole around it after it runs out of fuel. Many of these "seeds of the black hole" then merge into a much larger "supermassive black hole," which is located at the center of a known massive galaxy. There could even be a massive black hole at the center of the galaxy, surrounded by a large number of smaller but still massive "seeds" of huge stars. So far, astronomers have identified only a handful of intermediate black holes with mass in the Milky Way. They are not as powerful as a supermassive black hole, but they are much harder to find than the massive ones at the center of the galaxy. Black holes are generally defined as those in which gravity pulls so strongly that not even light can penetrate. There is a region around the black hole where intense gravity tears matter to shreds, but matter does not fall into it. This uniqueness forms the center of the black hole and is obscured by objects on the surface above or below the event horizon. The event horizons will likely be surrounded by a bright, incredibly high-energy material swirling around the black hole. This would take the form of a supermassive black hole with a mass more than 1000 times that of the Sun. Every second it creates an orbit that can be reached in less than a millisecond, and that would cause a massive explosion of energy and radiation.