What would be in the middle of a galaxy?

There are two things in the center of a typical galaxy.

First, there is a much higher density of stars, and gas as well. This is why the center of a galaxy tends to be brighter. This inner region of higher stellar density is called a "galactic bulge". In elliptical galaxies, which don't have spiral arm structures and instead have a more uniform appearance there is still an increase in density toward the center.

Second, most large galaxies seem to have a supermassive black hole at their center. Under the right conditions if there is a lot of matter falling into such a black hole then it can give rise to extremely energetic phenomena. Depending on the details such phenomena are given different names, such as active galactic nuclei (AGNs) or quasars or blazars (which are now understood to be subsets of AGNs). These phenomena can sometimes outshine the light from all of the other stars in the galaxy, sometimes by orders of magnitude, but they are relatively uncommon and also usually depend on a fortuitous viewing angle.