Why do Carolyn Crawford and Carlos Frenk say dark matter cannot be ordinary matter?

Carolyn Crawford said the first suspects were dim ordinary objects such as planets, gas clouds, brown dwarfs and black holes, known as baryonic matter. Any of them warmer than absolute zero would emit radiation, and in the quantities needed today's detectors would have seen the glow. Carlos Frenk added the clinching argument: the cosmic microwave background measures the amount of ordinary matter to an accuracy of about 1 per cent, and it falls far short of the total mass.

• The microwave background was discovered in 1964 by Penzias and Wilson and has cooled to 2.7 degrees above absolute zero.
• Its pattern of tiny hot and cold spots records what the young universe contained, about 350,000 years after the Big Bang.

▸In their words

“It's a really precise measurement with an accuracy of 1%.” Carlos Frenk · at 27:48 — 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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