How do astronomers know dark matter exists if it gives off no light, according to In Our Time?

On In Our Time, Carolyn Crawford, Anne Green and Carlos Frenk explained that dark matter is inferred from its gravity on things that do shine. Galaxies in clusters move too fast to stay together, stars in the outer Milky Way orbit as fast as inner ones, and hot gas in clusters should have escaped. Crawford stressed that different telescopes, wavebands and assumptions all point to the same missing mass.

• Fritz Zwicky measured cluster galaxies moving at around 1,000 km per second in the 1930s.
• Rotation curves measured in the 1970s showed stars at a few hundred km per second regardless of distance from the centre.
• X-ray telescopes found about ten times more hot gas than galaxies in clusters, yet still five or six times too little mass.

Source: our own transcription of the episode. The full conversation is on In Our Time (2026-10-26). Read the whole episode summary →

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