A lot of things depending on what kind of galaxy you are talking about. All galaxies seem to be shaped like gravitational wells into which gas and dust and stars sink further and further over the course of billions of years. In the nucleus, there may be an almost continual 'rain' of gas from outside the nucleus, which can settle into a disk-shaped, rotation maelstrom. We see such flattened disks of gas and dust in many galaxies including our own. What happens inside this disk depends on a lot of factors, but it seems common enough that stars can be formed, supernovae produced in tremendous numbers, and even supermassive black holes can be created there. All of this happens within a dense clustering of billions of stars which fill the sky far more densely than anything we can imagine way out here in the boondocks of our own galaxy. It must be both a glorious view, and a terrifying thought that your next door neighbor might be a monstrous black hole just waiting for your little star to plunge into its deathly maw!