Seeing the shadow of a black hole Why can't we see the shadow of a black hole cast on Earth. Shouldn't the black hole block some light from a star or a galaxy far behind as they line up?
 A: Yes, it will block some light. But barely anyone would notice if Gliese 581 were suddenly covered up by a black hole, and it's only 20 light-years away. The closest known black hole is 1600 light years away. You'd need some pretty sharp vision.
Also, you're going on the assumption that black holes block light like normal objects, which they don't. They do it like this: https://en.wikipedia.org/wiki/Gravitational_lens#mediaviewer/File:Black_hole_lensing_web.gif
Black holes are weird :)
A: One of the reasons is that very little light actually gets to earth from this galaxy or star, so the change in the amount of light coming in is negligible.  You don't "see" any shadow because there was almost no light coming to the earth in the first place.
The next reason, the more important one, is that the light from other galaxies/stars fill in the shadow that would be created by the black hole.  For instance, if that Galaxy were the only source of light, then technically the black hole would cast a shadow, but in reality light from other stars fills in that shadow.
