Modern gravitational theories predict the existence of black holes, regions of space where the gravity is so strong that nothing can escape from them——not even light. Because light can't escape, black holes are invisible and we can't directly see them. However, there is strong, indirect observational evidence of their existence. As matter near a black hole in the center of a galaxy swirls inwards, strong electromagnetic forces push some plasma away from the black hole. This force causes the matter to form powerful jets. These jets, and stars that orbit with increasing speeds, are what lead scientists to believe that black holes do indeed exist.

Images, taken by a European observatory's very large telescope in Chile, of the very center of our Galaxy show stars orbiting around a point at which there is a black hole of 2.6 million solar masses. Also, recent observations by the Hubble Space Telescope have shown that stars close to the center of the galaxy M32 are moving much faster than expected. Their motions can only be explained by the presence of a supermassive black hole in the center of that galaxy.

Collaged image of merging galaxies NGC4038 and NGC4039. Small green outline at left corresponds to area on the right of a higher resolution image by Hubble Space Telescope—Courtesy of Space Telescope Science Institute & NASA.

This image of NGC4038 and NGC4039 shows two spiral galaxies, known as the "antennae" galaxies, that are in the process of colliding with each other. When galaxies collide, the supermassive black holes at their centers have the opportunity to interact and merge. Images like this are evidence that galaxies do collide, thus providing an opportunity for the supermassive black holes at their centers to merge. Merging supermassive black holes are expected to be among the strongest sources that LISA will be able to detect.

On to Black Holes II