THE mysterious substance dark matter could be shining with its own kind of light. While this "dark radiation" would be invisible to us, it could have visible effects. Astronomers usually assume that dark matter particles barely interact with each other. Lotty Ackerman and colleagues at the California Institute of Technology in Pasadena decided to test this assumption by supposing there is a force between dark matter particles that behaves in the same way as the electromagnetic force. That would imply a new form of radiation that is only accessible to dark matter. Their calculations showed that it could have as much as 1 per cent of the strength of the electromagnetic force and not conflict with any observations