Black holes

It is believed that all objects with mass can become black holes as long as their radius is reduced to a certain size, even individual particles can become black holes. In a book written by professor Brian Cox it pointed out that if the Earth were compressed into the size of a pepple, a few centimetres across, it would be a black hole. 

The same book pointed out that gravity is the weakest force, a small magnet can pull a metal object away from the Earth, but it has no limits, it keeps increasing with mass and can be focused into a small space and cause a sphere around itself with an escape velocity that is higher than the speed of light. In other words, even light can't escape the gravity of a black hole if it gets too close. They call the space where light can't escape the event horizon.