the theory says in the center of each galaxy there must be a blackhole with an incredibly dense mass concentrated in a very small space. We can't "see" them because light that is entering a blackhole simply "stops" making it impossible to see what's inside because the outside is covered by the light that got trapped.
The center of the galaxy is too far away for us to see what's there.
All of those that have not been observed.
First, the center of a galaxy is not a "bright light". It's a mathematical point. This is a technical quibble, and it is true that the centers of spiral galaxies (like the Milky Way) tend to be brighter than the rest of the disk due to the density of the stars there. The reason we don't see the center of our own galaxy as a particularly bright patch is because we don't see by radio waves. In the radio spectrum, the center of the galaxy is quite bright. However, we see by visible light, and there's a lot of dust and gas between us and the center of the galaxy, which blocks out most of the wavelengths we can see.
You are looking through one of the spiral arms of our galaxy (the "Orion Arm", I think, but not 100% sure on that) toward the center of the galaxy. We cannot see the center of the galaxy with the naked eye because there is 30,000 light years worth of gas and dust in the way, but infrared photography (in which the gas and dust are transparent) shows it clearly.
In science, there are no proofs, per se. You can only disprove. Generally, when speaking of the center of the galaxy, one refers to the center of the galactic mass. A galaxy revolves around its center of mass, and we can see a galactic bulge in its center. We know this from observations of other galaxies, and Doppler images of its motion at the edges, when seen edge-on, one side will be slightly red-shifted relative to the other. In our galaxy, by mapping the known stars in a three-dimensional grid, we can see a central bulge and spiral arms, similar to what is seen in many other galaxies. This is sufficient to know that we are in an arm of the galaxy, about 2/3 of the way from the center to the rim.
We cannot see the center of the Milky Way galaxy, because it is hidden by clouds of interstellar gas. However, there is reason to believe that there are no stars at the center of the Milky Way, but only a supermassive black hole.
No. All the stars you see at night are in our galaxy, but outside of the solar system. The only star in our solar system is the one at its center: the sun.
Impossible to know because we don't know EVERY galaxy. The portion of the universe that we have to ability to see is extremely small compared to the entire universe.
A very large amount of darkness
Approximately 20 times. See related link and question
Because interstellar dust blocks any chance of visual observations.