Why were neutrinos ruled out as dark matter, according to Carolyn Crawford and Carlos Frenk?

Carolyn Crawford said neutrinos looked promising because they are real, plentiful and uncharged, but experiments put their mass too low, and being light they move close to the speed of light. Such fast hot dark matter would build huge structures first, while the real universe grew from small structures up. Carlos Frenk said his first 1980s simulations of a neutrino universe looked nothing like ours, his first big disappointment as a scientist.

• Frenk then set out to rule out cold dark matter and, about 35 years later, still could not.
• Cold dark matter simulations predicted the cosmic web of galaxy filaments later found in galaxy surveys.
• Warm dark matter remains possible and is hard to tell apart from cold.

▸In their words

“So the point about neutrinos is they travel too fast.” Carolyn Crawford · at 34:14 — open in the transcript →

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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