I probably could give a detailed answer as astronomy is not my area of expertise! Galaxies are still forming, the first ones probably soon after the Big Bang and they form by matter(particles) being pulled together.
Condensation of matter under the force of gravity is the driver here. When the universe formed after the big bang everything was a lot closer together, most galaxies were probably begun then.
Galaxies are thought to form around supermassive black holes, which have an enormously strong gravitational force that holds a galaxy of stars and planets together, separate from the rest of the universe. The bigger the black hole, the stronger the gravity, and the bigger the galaxy.
I’ve tried to read up on how supermassive black holes formed, but I’m afraid I don’t understand it at all! This is really quite different from the kind of science I do. All I learned was that black holes form when a LOT of mass comes together – when huge stars burn out, or explode, and combine with each other. That means that before you could form a galaxy, you’d first have to form stars, until more and more of these stars came together to form a supermassive black hole, strong enough to pull together a galaxy. Have a look at this page on Wikipedia that talks about galaxies – maybe you’ll learn more than I did! http://en.wikipedia.org/wiki/Galaxy
when the big bang first occurred, there were no galaxies.
They only started to form when parts of the universe cooled down and hydrogen, helium and ‘dark matter’ started to clump together (we still don’t know what dark matter is, but know its there!).
As these gases cooled, they formed the first stars and hence the first galaxies. The universe was only 500 million years old at this point, but things speeded up after that and early space must have been a very violent, spectacular place !
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