Black holes are the remnants of a super nova and are found in the middle of galaxies. Black holes have the strongest gravity of anything in the universe. They have such a strong gravity that at a certain point even light can not escape from it and the speed of light is the fastest speed there is. If someone would be sucked in to a black hole, he would feel comfortable until gravity is much stronger until he would be stretched. Scientist call that Spaghettification. What someone on the outside would see is the person who got sucked in move slower and slower, until they cross the point were light cant escape. After that the person would slowly vanish.