Dark Matter and Dark Energy

This lesson covers dark matter and dark energy — two of the most mysterious and important concepts in modern physics — as required by the AQA GCSE Physics specification (4.8.2). This is a Physics-only topic. You need to understand why scientists believe dark matter and dark energy exist, what evidence supports their existence, and how they affect the universe.

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The Missing Mass Problem

In the 1930s, the Swiss astronomer Fritz Zwicky studied clusters of galaxies and noticed something puzzling. When he calculated the total mass of a galaxy cluster by adding up the masses of all the visible stars, gas, and dust, the total was far too small to account for the gravitational forces holding the cluster together. The galaxies were moving so fast that they should have flown apart — yet the cluster remained bound together.

Zwicky proposed that there must be a large amount of invisible matter providing the extra gravitational pull needed to hold the cluster together. He called this unseen material "dark matter" (dunkle Materie in German).