the creation of the galaxies

[lugal-demon-lord-king-0]

why do most galaxies have a spiral shape?also i heard that most galaxies have a supermassive black hole in the center, so i was wondering how does that work and is a black hole part of the creation of the galaxies?  

[Sioga]

your answer comes in three parts.

1. only some galaxies are spirals, there are other types of galaxies.
2. galaxies are formed by particles that are pulled together by gravity. the particles spiral around each other, creating the stars, planets, asteroids, etc and those become the galaxy.
3. the black hole is created when a lot of those particles are super-compacted and become a very dense material, aka, the black hole. although the black hole theory is still being debated because we cant exactly see the center of the galaxy.

does that help any?  

[Layra-chan]

There isn't a fully accepted theory of how galaxies form yet. Some astrophysicists think it's a bottom-up process, wherein small clusters of form, and then those clusters group together, and then those groups themselves group together, etc. until a galaxy appears. Others think that it's a top-down process, wherein a huge glob of matter coalesces and then collapses all at once (all at once meaning over millions of years), forming all the stars in roughly a single event.
Supermassive black holes regulate the distribution of matter in newly forming galaxies, pulling the farther stars inwards and maintaining the angular momentum of the galaxy.

As for why a lot of galaxies are in a spiral shape, a spiral shape is a very stable shape for that many moving stars. The spiral arms can be thought of as density waves (like sound waves) where the density of the stars changes with the waves, even if the stars don't move along with the wave (just like how sounds can travel through the air even if the individual air molecules don't move very far). The stars in the middle of the galaxy are orbiting the center of the galaxy faster than than the stars at the far reaches, and stars pull at each other, so that the stars in the middle orbit, pulling the stars slightly farther out along with them, which pull the stars slightly more farther out along, and so on, so the density waves curl instead of fanning straight out from the center. Hence we get a spiral shape.

A lot of galaxies actually aren't in a spiral shape; the other common shape is elliptical. The spiral shape makes sense if the angular momentum is fairly uniform throughout the galaxy, but otherwise they tend to just get pulled into roughly what you'd expect by gravity.  

[lugal-demon-lord-king-0]

well on the followup, will the acceleration(dark matter/energy) change the shape of the galaxies and to what?  

[Layra-chan]

Probably not to any significant extent. Individual galaxies exist on a scale far too small for the expansion of the universe to have a noticeable effect. Unless the expansion rate increases a lot, the expansion isn't significant at scales smaller than galactic groups and clusters.