In the 41st Millenium, what's in the centre of the Galaxy?

Simple question really, I was wondering if anyone has read any fluff about Mankind expanding into the centre of the galaxy and discovering anything. After all Terra is off to one side and we have nids attacking the east and south but what's going on in the really bright middle bit? It's not a serious question i.e. blackhole, modern astro-physics theory etc just a purely fluff-based question e.g. any gribblies/chaos god/Uber-C'Tan/The Big E's mum and dad etc