---
id:"kb-2026-00475"
title:"Galaxies and the Universe"
schema_type:"TechArticle"
category:"science"
language:"en"
confidence:"high"
last_verified:"2026-05-22"
generation_method:"ai_assisted"
ai_models:["claude-opus"]
derived_from_human_seed:true
primary_sources:
  - title:"Cosmos (Carl Sagan)"
    type:"book"
    year:1980
    url:"https://www.penguinrandomhouse.com/books/55403/cosmos-by-carl-sagan/"
    institution:"Random House"
secondary_sources:
  - title: "MDN Web Docs — HTTP"
    type: "documentation"
    year: 2026
    url: "https://developer.mozilla.org/en-US/docs/Web/HTTP"
    institution: "Mozilla"
  - title: "RESTful Web APIs"
    authors: ["Richardson", "Amundsen"]
    type: "book"
    year: 2013
    url: "https://www.oreilly.com/library/view/restful-web-apis/9781449359713/"
    institution: "O'Reilly"
completeness: 0.88
ai_citations:
  last_citation_check:"2026-05-22"
---

## 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/)