Galaxies are collections of millions (or sometimes even hundreds of trillions) of stars, plus all the things that orbit those stars, plus the particles of the interstellar medium, and (in theory) large amounts of dark matter. These collections orbit around their collective center of mass, held together by gravity. Many galaxies are believed to have a supermassive black hole at the center.
Sci Fi writers have no sense of scale, often using the term galaxy to mean planetary system or vice-versa, especially in older works of science fiction. This is something akin to accidentally using the word "nation" when you mean to say "neighborhood". It's grossly mis-stating the size and scale of the community you're discussing. At the other end of the spectrum, they also sometimes erroneously use galaxy to mean universe, or vice-versa.
Scientists estimate that there are at least 170 billion galaxies in the known Universe. Galaxies are surrounded by a great nearly-empty void, commonly known as intergalactic space. Despite this void and the immense distances it covers, galaxies are rarely isolated entirely from one another. 95% of galaxies belong to galaxy groups or even superclusters. Typically the distance between galaxies within a cluster is roughly 10 times the size of the galaxies themselves. Sometimes galaxies collide or pass very near one another, which can deform them or even cause them to merge into one larger galaxy.
By far the most famous or familiar galaxy is our own Milky Way, which includes the Earth and the Sun, as well as the rest of the Solar System. It also includes over 200 billion additional stars (and all the planets, etc, that orbit them). The milky way is a multi-limbed spiral 100,000 light-years long, and 3,000 light-years tall at it's widest part, making it a little larger than average. At the heart of our galaxy is a supermassive black hole known as Sagittarius A*. Our galaxy is part of the Local Cluster of galaxies which includes the Andromeda Galaxy and dozens of smaller dwarf galaxies. This local cluster is in turn part of the Virgo Supercluster, which is made up of hundreds of galaxies.
- Active Galaxy / Active Galactic Nucleus
- Barred Spiral Galaxy
- Dark Galaxy
- Dark Matter Halo
- Dwarf Galaxy
- Elliptical Galaxy
- Interacting Galaxy
- Interstellar Terrain
- Irregular Galaxy
- Physicists Find Evidence That The Universe Is A 'Giant Brain'
- Radio Galaxy
- Ring Galaxy
- Spiral Galaxy
- Starburst Galaxy
Game and Story Use
- Other (or multiple) galaxies should logically be beyond the scale and scope of a game or most works of fiction, unless you're dealing with sufficiently advanced science. Exploring your own galaxy is a hugely daunting task that will take enormous amounts of time even given some sort of FTL travel. Going beyond it to the next galaxy over is at least 10 times as much distance/time/energy/hassle as it would take to cross from one end to the other of your own galaxy. It takes 2.5 million years for light to reach us from the "nearby" Andromeda Galaxy.
- That said, if your Space Opera -style game features a myriad of races forming more or less a single galactic civilization, you might make use of threats that come from another galaxy. If your own galaxy is so well-explored that it's become mundane, you may need something from beyond its borders to feel truly alien.
- While transit between galaxies is unlikely to be a key feature of any game striving for realism, it's certainly possible to set your game in an entirely different galaxy, long ago and far, far away. Some of the more peculiar galaxy types (ring galaxies, for example) could make for an interesting shape for your map of known space.
- Intentionally misusing the word galaxy may help build the flavor of a campy retro-sci-fi game. If your game is set in the future as old black and white matinee serials imagined it to be, then galaxies could be nations, and consist of only a few habitable worlds each. Science be damned!
- "You pitiful Earth-men are powerless against our mighty spaceship. Your entire Earth Galaxy shall be destroyed, all 9 planets vaporized!"
- "We're all out of milk, so I'll take my rocketship to the convenience store — it's just two galaxies away!"