---
title: "Dark Matter, Dark Energy and What the Universe Is Made Of"
description: "Galaxy rotation curves and cluster masses need far more matter than shines, and the expansion is speeding up rather than slowing. Ordinary matter turns out to be a small part of the total."
canonical: https://lightmysky.com/learn/science/dark-matter-dark-energy-and-what-the-universe-is-made-of-mt_GKKPGcHqJH
source: https://lightmysky.com/learn/science/dark-matter-dark-energy-and-what-the-universe-is-made-of-mt_GKKPGcHqJH.md
retrieved: 2026-09-12
---

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# Dark Matter, Dark Energy and What the Universe Is Made Of

Galaxy rotation curves and cluster masses need far more matter than shines, and the expansion is speeding up rather than slowing. Ordinary matter turns out to be a small part of the total.

Subject: Science · Area: Astronomy & Astrophysics · Ages 21 to 22
Page: https://lightmysky.com/learn/science/dark-matter-dark-energy-and-what-the-universe-is-made-of-mt_GKKPGcHqJH

## Ready when they can

- Explains what a flat rotation curve implies about mass beyond the visible disc
- Describes the supernova evidence that the expansion is accelerating
- States the rough proportions of ordinary matter, dark matter and dark energy

## Lesson: The universe we cannot see

Galaxies spin too fast for the gravity of their visible stars and gas. Rotation speeds stay high far out in the disk instead of falling off, which means unseen mass surrounds every spiral in a vast halo. Clusters agree: member galaxies move so quickly that only five to ten times more mass than we see could hold the group together.

That missing mass is dark matter: slow moving, neutral, and almost non interacting, mapped by galaxy motions and by lensing. Decades of searches have not caught a single particle, yet the gravitational case keeps strengthening. Lensing maps the extra mass directly and agrees with the motions.

**Example.** Distant supernovae looked dimmer than their redshifts allowed, which means the expansion is speeding up rather than slowing. Something pushes outward on cosmic scales, and that push is called dark energy. Acceleration was the surprise of the late 1990s.

**Tip.** Remember the rough shares of the critical density: about 5 percent ordinary matter, about 27 percent dark matter, and about 68 percent dark energy. Only half of one percent sits in stars. Nearly everything out there neither shines nor has been caught.

**Recap.** Fast orbits reveal dark matter, dim supernovae reveal dark energy, and ordinary matter is only about a twentieth.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Cosmic Microwave Background and the Light Elements](https://lightmysky.com/learn/science/the-cosmic-microwave-background-and-the-light-elements-mt_oz0MJXiuVt)