---
id: kb-2026-00475
title: Galaxies and the Universe
schema_type: TechArticle
category: science
language: en
confidence: medium
last_verified: "2026-05-28"
created_date: "2026-05-22"
generation_method: ai_assisted
ai_models:
  - claude-opus
derived_from_human_seed: true
conflict_of_interest: none_declared
is_live_document: false
data_period: static
atomic_facts:
  - id: af-science-galaxies-and-the-universe-1
    statement: >-
      NASA describes galaxies as large systems of stars, gas, dust, and dark matter held together by
      gravity.
    source_title: Galaxies
    source_url: https://science.nasa.gov/universe/galaxies
    confidence: medium
  - id: af-science-galaxies-and-the-universe-2
    statement: >-
      NASA states that the Milky Way probably contains 100 to 400 billion stars and is about 100,000
      light-years across.
    source_title: "Our Milky Way Galaxy: How Big is Space?"
    source_url: https://science.nasa.gov/universe/exoplanets/our-milky-way-galaxy-how-big-is-space/
    confidence: medium
  - id: af-science-galaxies-and-the-universe-3
    statement: >-
      NASA Hubble measurements predicted a future collision between the Milky Way and Andromeda
      galaxies in about 4 billion years.
    source_title: NASA Hubble Shows Milky Way is Destined for Head-on Collision with Andromeda Galaxy
    source_url: >-
      https://science.nasa.gov/missions/hubble/nasas-hubble-shows-milky-way-is-destined-for-head-on-collision-with-andromeda-galaxy
    confidence: medium
completeness: 0.88
known_gaps:
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    Certain sub-topics are covered at a general level; specialized edge cases and nuanced
    applications may not be fully addressed
disputed_statements: []
primary_sources:
  - id: ps-galaxies-and-the-universe-1
    title: Galaxies
    type: government_resource
    year: 2026
    institution: NASA Science
    url: https://science.nasa.gov/universe/galaxies
  - id: ps-galaxies-and-the-universe-2
    title: "Our Milky Way Galaxy: How Big is Space?"
    type: government_resource
    year: 2018
    institution: NASA Science
    url: https://science.nasa.gov/universe/exoplanets/our-milky-way-galaxy-how-big-is-space/
  - id: ps-galaxies-and-the-universe-3
    title: NASA Hubble Shows Milky Way is Destined for Head-on Collision with Andromeda Galaxy
    type: government_resource
    year: 2012
    institution: NASA Science
    url: >-
      https://science.nasa.gov/missions/hubble/nasas-hubble-shows-milky-way-is-destined-for-head-on-collision-with-andromeda-galaxy
secondary_sources: []
updated: "2026-05-28"
---


## 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/)

## Related Articles

- [Astronomy and Cosmology: From Solar System to the Universe](../astronomy-and-cosmology.md)
- [The Big Bang: Cosmic Microwave Background and the Origin of the Universe](../the-big-bang-cosmic-microwave-background-and-the-origin-of-the-universe.md)