What is a galaxy?
One-line takeaway
BLUF: A galaxy is a massive, gravitationally-bound system of stars, gas, dust, dark matter, and other components.
A fundamental explanation of what is a galaxy?
The Explanation
A galaxy is a massive, gravitationally-bound system of stars, gas, dust, dark matter, and other components. It can contain from millions to trillions of stars (the Milky Way has ~10^11), interstellar gas and dust clouds, and typically a dark-matter halo. Galaxies come in shapes (spiral, elliptical, irregular) and often host a supermassive black hole at their centers. They are the fundamental "islands" of stars in the universe. Each galaxy orbits collectively under its own gravity, and galaxies themselves group into clusters and superclusters. Thus a galaxy is a vast cosmic community of stars and matter held together by gravity.