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
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '