If most of the large galaxies have black holes at the center, why do photos show a very bright spot instead of a black spot?
There are 2 explanations for the bright spot apparent at the center of large galaxies: one local to the center and one due to our perspective at a distance.
Locally, there is a large amount of gas and dust attracted by the black hole's gravity. As this material approaches the black hole, it swirls in a vortex, like water swirling down a drain. As the material is packed together and falls into the hole ( passing inside the Swarzchild radius when we can no longer see what happens to the material) it emits large amounts of radiation. This, plus re-emitted light as this radiation heats up surrounding gas and dust, is visible as a bright shell or disk around the black hole.
However most of the brightness apparent at the center of a large galaxy is not at the very center. Galaxies are huge, and the density of stars tends to be greater towards the center of the galaxy. Therefore a line of sight towards the center of the galaxy will likely contain thousands of stars and therefore appear bright.
Rob Landolfi, Science Teacher, Washington, DC
'There must be no barriers for freedom of inquiry. There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors.'