Tiny black holes, born in the first moments after the Big Bang, should have disappeared long ago according to Hawking's theory — evaporated like a puddle of water under the sun. But new calculations show evaporation isn't that fast. The hole seems to choke on its own radiation — like a boiling pot covered by a cloud of steam, slowing the boil. This 'memory burden' effect turns an instantaneous flash into a long, slow fading.
Such 'underboiled' holes spent eons warming the surrounding hydrogen in the young universe. Hydrogen atoms, feeling the heat, change their radio voice — 21-cm radiation. Astronomers catch this ancient signal and see it's too cold. If black holes were more than a tiny fraction, they would have heated the gas more, distorting the radio picture. So these objects cannot be dark matter — their number is less than one ten-millionth of the required amount.
The most sensitive tool for hunting dark matter isn't a giant underground detector, but an ordinary radio telescope listening to the echo of hydrogen from the time before the first stars lit up.