## TL;DR

A galaxy is a gravitationally bound system of stars, gas, dust, and dark matter. Milky Way: ~100-400 billion stars, 100,000 light-years across, spiral type. Andromeda: nearest major galaxy, will collide with Milky Way in ~4.5 billion years. Observable universe: ~93 billion light-years across, 2 trillion+ galaxies.

## Core Explanation

Galaxy types: spiral (Milky Way), elliptical, irregular. Supermassive black hole: center of most galaxies (Sagittarius A*, 4 million solar masses). Dark matter: ~27% of universe, invisible, detected via gravitational effects. Dark energy: ~68%, causing accelerating expansion. Hubble Deep Field (1995): pointed at empty patch for 10 days → thousands of galaxies. James Webb Space Telescope (2021+): sees earliest galaxies (~300M years after Big Bang).

## Further Reading

- [Cosmos (Carl Sagan)](https://www.penguinrandomhouse.com/books/55403/cosmos-by-carl-sagan/)