Black holes.

A black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape from. The term "Black Hole" comes from the fact that, at a certain point, even electromagnetic radiation (e.g. visible light) is unable to break away from the attraction of them. This renders the hole's interior invisible or, rather, black like the appearance of space itself.

Despite its interior being invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit outside its event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. It´s like, you see a star today and then...PUFF! It´s gone, but where to? Then you look closer and see it´s been happening with a lot of elements. A-HA! There may be a black hole there.