the box. Thanks for your patience as we answer your questions. mkjohnst: I've been reading about how scientists like you can use gravitational lensing to weigh clusters of galaxies, and that the lensing effects indicate a lot more matter than is visible, apparently evidence for dark matter. But how do you distinguish between dark matter and black holes in exercises like this? riyanatarajan: The distribution of dark matter that is inferred from gravitational lensing studies suggests that it is distributed quite smoothly throughout a galaxy - from the inner regions all the way to the outskirts or the halo. While black holes might constitute one form of dark matter, they cannot account for all the dark matter in the Universe (90% of all the matter in the Universe is dark) or even what is inferred in individual galaxies. Mr.Nice: how big is a black hole itself?and how big is the event horizon...generaly speaking Ezequiel: It depends on the mass of the black hole. For example, for the mass of the sun, the event horizon is 3 kilometers. And then, the size of the event horizon is proportional to the mass of the black hole. Kevin: There aren't any black holes that we know that are anywhere near being dangerous. Jason (moderator): For a good primer on the basics of black holes, check out: http://science.nasa.gov/astrophysics/foc\\... MooCow: what are the chances of the Voyager 1 and 2 being eaten by an undiscovered black hole? Kevin: Very very VERY low... Jason (moderator): Do