Some stars are gigantic compared to others. When a massive star like this cools and collapses, it turns into something very special. The matter is crushed and compressed by the huge weight of the collapsing gases. Gravity keeps squeezing and squeezing until the star seems to disappear. It has become a black hole. Anything passing too close to a black hole - even light rays - will be pulled into it and never escape again. That is why a black hole is invisible. Scientists, however, are certain that they have located several black holes in the universe.