When a massive star reaches the end of its lifetime, it explodes as a Supernova. The outer layers of the star are blown off and only the cold core of the star is left. This remnant core of the massive star gets so much compressed under its own gravitational pull that an extremely dense object is formed which is called the Black Hole. Because of the extreme density of a Black Hole, the Black Hole’s gravitational force of attraction is so strong that even the light is unable to escape from it. As light fails to come out from the Black Holes, they cannot be seen by our eyes.