system
Given the following post, respond as if you're commenting on Reddit:
human
When describing black holes in galaxies people generally say “almost all galaxies have black holes at their centers” why almost”? Are there galaxies without supermassive black holes in their centers, if so how do they form?
gpt
Supermassive black holes aren't critical for the formation of galaxies - they're just a small part of them.

Galaxies start to form when dark matter collapses into a "halo" - a big ball. The gravity of this concentration of dark matter causes gas to follow along and collapse into the centre of this halo. The gas then cools, fragments, and collapses into stars, forming a galaxy. This is a continual process, as more gas and dark matter falls in from intergalactic space, and as galaxies merge, combining their gas, stars, and dark matter. The gas is also not used up immediately - the stars heat the gas up, providing pressure to stop the gas from collapsing immediately into more stars. So you get a feedback effect - more gas means more stars get formed, but more stars means that you slow down the formation of new stars, and you get a slow burn of star formation.

The greatest concentration of gas and stars in a galaxy is typically in the centre. The combined gravity of the gas, stars, and dark matter mean that the centre is essentially "downhill" - matter naturally tends to drift that way. Somehow, the central concentration of gas & stars forms a supermassive black hole.

It's not entirely known exactly which way the central concentration of *stuff* turns into a black hole. It may be that you have a bunch of massive stars in a central star cluster that turn into a number of black holes that merge into a massive black hole. It might be that the big pile of gas just directly collapses and forms one huge black hole. Or something else along those lines. It's still difficult to test which one of these is the correct scenario. We have the ingredients - a concentration of stuff in the middle - but it's not clear which out of the many possible ways this could collapse into a black hole is correct. It may be that several of them work in different environments. But once you have a "seed" black hole of enough mass, we understand how it can continue to pick up mass and grow to be supermassive.

So it's not the case that you have a supermassive black hole and a galaxy forms around it. It's the opposite - a galaxy forms a concentration of gas and stars in its centre, and this may or may not form a supermassive black hole, somehow. In particular, very small galaxies - dwarf galaxies - tend to be quite irregular, and don't always have well defined high density cores. They don't seem to have supermassive black holes in their centres, as far as we can tell, and we might not expect them to form them (although this is debated).

Additionally, the supermassive black holes usually only make up a tiny fraction of the galaxy's mass. For the Milky Way, the supermassive black hole (Sag A\*) has a mass of about 4 million times the mass of our Sun. However, there are over a hundred *billion* stars in our galaxy. And if you count the dark matter, the mass of our galaxy is about a *trillion* times the mass of the Sun. If you removed the supermassive black hole from the galaxy, it would have almost no measurable effect on our orbit.

The dramatic thing about supermassive black holes is not their mass, but their *energy output*. In some galaxies, the SMBH is actively accreting gas from the surrounding regions, and has an incredibly bright accretion disk around it. This gives off so much radiation that it heats quite a major fraction of the galaxy's gas, and the intense magnetic fields of the accretion disk can even fire jets thousands of light years away.

So, to summarise:

- galaxies form from the collapse of dark matter (and gas)

- SMBHs then form in the centre (massive) galaxies, somehow

- but SMBHs are a tiny fraction of a galaxy's mass, and aren't important for the orbits of most stars (although they can have other dramatic effects)