but was not proven. only 20 years ago it was proven. the existence of black holes in the center of our galaxy. and then we know the center of every galaxy is a black hole, and that's thanks to the great contributions of stephen hawkings. >> what does that mean, though? what does that matter to our understanding of the world and how it operates? >> well, they are massive because they concentrate such a great amount of mass in a small volume of space. so it's such a big concentration that it forms a singularity. singularity is also a word that was introduced i i believe by stephen hawkings. black holes to the point when it reached this level of concentration and singularity where the gravity force is so big, so huge that nothing escape