Black holes are one of the greatest mysteries of nature. They are nearly impossible to see with telescopes and other instruments because of their interaction with light. Black holes are so difficult to see that it would be difficult to see one if it were right beside the Earth. This paper will provide an overview of the mystery that is the black hole and provide a discussion of some of the main features of black holes including the causes of black holes, the characteristics of black holes, and an overview of some current research and discovery relating to black holes.According to hubblesite.org, the homepage for information gathered by the Hubble Telescope, black holes are areas in the universe where gravity is so overwhelming and strong that it pulls in all other forces within its event horizon. Gravity is so strong inside black holes that once something crosses the outer edge of the black hole, it cannot escape (hubblesite.org). Black holes are created by the collapse of a star. When an object as massive as a star collapses in on itself, the mass of the star becomes concentrated into what is known as a singularity or a single point in space. This single point is mind-bogglingly small and holds all the mass that was once in the star. It is so massive, yet so small that its gravitational force is strong enough to even prevent light—the fastest known force in existence—from escaping.The term “black hole” was not coined until 1967 when physicist John Wheeler was studying Einstein’s theories on general relativity “which showed that when a massive star dies, it leaves behind a small, dense remnant core” whose mass must be more than at least three times the mass of our sun (science.nasa.gov). Because even light cannot escape a black hole, scientists are unable to observe black holes directly and must resort to indirect measurement to observe the supermassive object.Without the aid of telescopes that measure light, x-rays, or electromagnetic radiation, black holes can only be seen by measuring their effect on nearby objects like other stars or clouds of interstellar matter. Through a process called “accretion,” black holes draw matter inward and this can be measured and seen by scientists who can see the x-rays emitted by objects as they are torn apart by a black hole.As mentioned previously, black holes are often the result of gravity causing a star to collapse inward on its self. Black holes, which result from massive stars, cannot burn nuclear fuel like a normal star and cannot support themselves because they can no longer generate thermal pressure (Teukolsky 1). According to Nasa, “as a star collapses…as the surface of the star nears an imaginary surface called the event horizon, time on the star slows to relative to the time kept by observers far away” and, as the surface reaches this event horizon, the star becomes stuck in time as a “frozen collapsing object” from the point of view of the observers (science.nasa.gov). Black holes like these can also result from the collision of large stellar objects like two stars colliding or two galaxies colliding or even from the collision of a black hole and a neutron star as was observed in 1994 by the Hubble Telescope (science.nasa.gov).According to hubblesite.org, all black holes are unique and often look very different from each other yet remain fundamentally identical. This is because of the black hole’s surroundings. Black holes can be characterized by three basic characteristics: mass, spin, and electric charge. It is the uniqueness granted each black hole by its surroundings that makes them measurable at all. Mass is measured by studying the orbits of surrounding objects. It is also likely that black holes spin in much the same manner as the stars that they once were. However, “no consensus has yet emerged” as to whether they do actually spin because this is very difficult to observe (hubblesite.org).One of the larger mysteries about black holes is what is exactly inside them? “We cannot glimpse what lies inside the event horizon of a black hole because light or material from there can never reach us” (hubblesite.org). According to current theories on the matter, all matter from the original star as well as all matter sucked in by the black hole is concentrated into a singularity as discussed previously. However, scientists are unsure as to exactly how this must work. For the most part, scientists agree that this must have something to do with quantum mechanics and something called quantum gravity. Although the phenomenon has a name, many scientists disagree as whether it actually exists.The conjecture related to the true nature of black holes is very interesting and sounds like something out of a science fiction novel or movie. Based on Einstein’s theory of general relativity, it is possible that the singularities within black holes actually lead to a point somewhere else in the universe. This “bridge” is called a “wormhole” and actually looks like a three dimensional area where space is “folded” and a shorter path is created by the black hole to create a shorter distance between the two areas of space.Why are we studying black holes at all if most of them are very distant and the science and math behind understanding them is still at such a juvenile state? According to Nasa, because some black holes could be as small as an atom, one could go completely undetected somewhere quite close to the Earth and we would not know about it until it had absorbed enough mass to have an effect on our orbit or the orbits of surrounding masses (science.Nasa.gov). While it is unlikely that a black hole exists anywhere near earth, the knowledge we can gain about things such as quantum physics, quantum gravity, wormholes, and the fabric of space-time will be invaluable and could lead to further more discoveries. Who knows—maybe one day we will understand black holes well enough to use them as gateways or wormholes to more distant parts of the universe and perhaps we can learn to create them ourselves to suit our space-travelling purposes.