Originally posted by RKIF-DragonClaw:
Aren't black holes invisible?
Only at the singularity, which, depending on the black hole's charge and whether or not it is spinning, can be either a single point or a huge, thin donut shape (kind of like a hollow frisbee).
A black hole floating along in space will be invisible, technically, but its effects on everything around it can sometimes be seen. Firstly, black holes bend light from behind the viewpoint around them, so there is a light distortion around the gravity well with black at the centre (no light can escape from there). If there is gas or matter within reach of the black hole's gravity, an accretion disc will form, where material heats up and gives off radiation as it moves closer to the singularity.
In this example, we can see there is a (possible) black hole there, as its effects on the star next to it are obvious.