Stars, comets, planets, they are all visible when you look up past our atmosphere. What if I told you that there were objects up there that you could not see, but had more gravity than an object 500 time larger than our sun. Objects, that are capable of swallowing whole planets within a split second of time. An object that would steal light right out of thin-air. These objects are more powerful than anything man made. Black holes are the most popular unknown object in science.
A black hole is an area of space that has so much mass crammed in it that there is no way for an object to escape its gravitational pull. Pretend that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don't throw it too hard, it will rise for a while, but eventually the pick up of speed due to the planet's gravity will make it start to fall down again. If you threw the rock hard enough, you could make it escape the planet's gravity entirely. It would keep on rising forever.
The speed that you need to throw the rock in order that it just barely escapes the planet's gravity is called the 'escape velocity.' The escape velocity depends on the size of the planet. If the planet is extremely large, then its gravity is very strong, and the escape velocity is high. A smaller planet would have a smaller escape velocity. The Earth's escape velocity 25, 000 miles per hour, while the Moon's is only 5300 miles per hour. Now imagine an object with such a large concentration of mass in such a small radius that its escape velocity was greater than the speed of light. Because nothing can go faster than the speed of light, a beam of light would be pulled back by gravity and would be unable to escape the black hole.
Black holes are not visible from the Earth nor any other planet in our solar system, but Black holes have more gravity than any other object known to man. Your probably asking yourself how this can be? Its as simple as this; Black holes are formed from a large star that dies and then gives in to its own gravitational pull dragging in everything around it. The star becomes so small that it can no longer be seen, but as it gets smaller its gravitational pull becomes greater. It is like taking a piece of bread and molding it between your hands until it is as small as a pea. As you were making your piece of bread smaller you were adding more and more stuff to the center.
When you add more to the core of something it increases the gravitational pull.