What causes galaxies to have a bright center

just a random curiosity I have had that I can't seem to find in researching and also if anyone knows why they have a bright center another question I have is what material is the center made out of that makes it dense enough to cause the gravity to hold our galaxy from just flying apart

The bright center is in fact mostly about the concentration of stars. Its pretty densely packed up in there.

The center of most, if not all, spiral galaxies is a super massive black hole. Millions and even billions of suns worth of mass. They are NOT the reason galaxies don;t fly apart though. They are the reason inner galaxies don't fly apart, as a for instance, our sun is not feeling the gravitational effects of Sagittarius A Star (SgrA* Our very own super massive). It is feeling the effects of a daisy chain of gravity which hubs at SgrA*. Gravity is cumulative.

ok BLACK HOLES known as quasars give of huge amounts of radiations that glows in the visible light spectrum.
two. it's not due to gravity in the center of the galaxy that holds it together but dark matter. I'm guessing you already knew this and either wanted a confirmation or were unsettled by this explanation for some reason and decided to get a second opinion.

Super massive black holes at the center can emit light and other forms of radiation as they devourer stars.
There are also more stars closer together towards the center because of the black hole's gravitational attraction. Heat and friction between the stars can also emit light and heat.

I think it's because of the close concentration of stars at the galaxy's gravitational center.

Gravity all thoes planets are being pulled together and when u get to much stuff in one place it makes a black hole!