The whole idea of black holes is just a theory, but there are a few other theories based on the physical properties of the black hole. The idea of a body so massive that nothing could escape it, not even light, was mentioned in 1783 by a man named John Mitchell. Some important figures that have contributed to the ever growing pot of theories include the famed Stephen Hawking, Albert Einstein, and Robert Oppenheimer of the Manhattan Project. I have only covered a handful of their theories on this website, but there are many more to be found in scientific journals and textbooks.
A lot of people wonder what might happen if they were to stray too close to the supermassive objects. The answer has been debated plenty, but it seems most scientists can agree on a few things. First, you wouldn't even be able to survive a fall into a black hole. Because of the difference in gravity on your head and feet, you would be literally pulled apart before you even got close to the black hole. Let's say, theoretically, that you could some how stay together and enter the black hole. As you approach the event horizon, far away objects begin to appear distorted because the gravity is bending the light from them. When you reach the event horizon, which is the point where nothing can escape the gravitational pull, something extraordinary happens. Whichever direction you look along the event horizon, you would see the back of your head, but this only lasts for an instant before you are sucked closer to the singularity. Once you are past the horizon, you look out into outerspace and are amazed at what you see. The whole universe is racing by you, stars are forming and exploding, galaxies are colliding, because the light waves are being pulled past you at severely increased speeds. Then, all of a sudden, you become part of the singularity.
First of all, while they are all moving faster, you are moving painstakingly slow to them. As you get closer to the hole, it appears that you are moving slower and slower because the light waves coming from your body are having a harder time escaping the gravit of the hole. This goes on until it appears that you are stuck in time. Another phenomenon that would occur if you were to go down to just outside the event horizon for a while and then come back, everyone you see would have aged much faster than yourself. This is because time passes slower as you get closer to the singularity.
If you were to shrink the mass of our Sun to a size small enough, it would become a black hole. But how does this happen in nature? When a star runs out of fuels to burn, it collapses. Some average size stars, like the Sun, collapse into smaller stars called neutron stars, but the biggest stars collapse into black holes. These black holes contain at least three solar masses. There are believed to be supermassive black holes at the center of most galaxies, including the Milky Way.
A lot of science ficion movies and novels employ the use of "wormholes" for time travel, and most people think that they are just part of the writers' imaginations. Well, in theory these wormholes could actually exist. If the singularity of a black hole is rotating, mass falling towards it might not hit the center. The womrhole theory explains that you could fall into a rotating black hole and be spit out at another black hole called a "white hole." This could be in another part of the universe, a different universe, or even in another time. Now, nobody actually knows that wormholes exist, and even if they did, they would be extremely hard to travel through because of their instability.