system
Given the following post, respond as if you're commenting on Reddit:
human
Why are galactic centers always occupied by black holes? Are galaxies to black holes what accretion disks are to stars?

I recently heard from Dr Tyson that all galaxies (or at least the vast majority, he wasn't being super precise in this presentation) we have checked appear to have black holes in their centers. That got me wondering why that would be, and this galaxy-scale accretion disk is the best idea I've got. But I feel like I would have heard of that before if the answer was that simple, and that doesn't really fit with the way that black holes are formed themselves. So what's the deal?
gpt
Kind of, but it's a bit misleading.

In both cases, you have a rotating disc of gas, and matter from this disc does slowly drip onto the central object. But there's a couple of big differences here.

The biggest thing is that a supermassive black hole is really only a very tiny fraction of the mass of a galaxy. The central SMBH of the Milky Way is about 0.1% of the mass of the *stars* of the Milky Way. And these stars are only a fraction of the mass of the dark matter mass. This means that the galaxy rotates very differently to a stellar accretion disc. The galaxy has a "flat" rotation curve, because its gravity comes from mass that is spread out. The angular speed (degrees/million years) of a star's orbit drops fairly slowly with distance, and the linear speed (km/s) is actually flat over a lot of the galaxy's radius. For a stellar accretion disc, most of the mass is concentrated in the centre, so you get a sharper drop off of gravity with distance.

Because the SMBH is on such a small scale compared to the galaxy's gas disc, there's a lot going on between the galactic gas disc and the SMBH. The SMBH doesn't even need to spin the same way as the galaxy. The SMBH actually does have its own real accretion disc, that is small and thin and hot and bright. Around this is a thicker "torus" of warm dusty gas. Gas from the galaxy can flow into the "torus", which can then slowly flow onto the actual SMBH accretion disc, and then onto the SMBH itself. 

But it is true that gas does tend to build up in the centre of a galaxy, and that's part of why you expect to find an SMBH there. You've got a lot of gas there in the early universe, and this either directly forms an SMBH seed (i.e. an intermediate mass black hole), or forms into massive stars that then turn into black holes and merge to form an SMBH seed (which route is correct is still fairly recent active research). The other thing is that massive compact objects tend to "sink" to the centre of a galaxy, because they lose kinetic energy by scattering nearby stars around. So even if a galaxy gets hassled up by a passing galaxy's gravity, or if two galaxies merge in a messy romance, the SMBH (or SMBHs!) will tend to settle down to the centre of the resulting galaxy in the end anyway.