Mysterious dark matter could be shining with its own private kind of light. This "dark radiation" would be invisible to us, but could still have visible effects.

Astronomers usually assume that dark matter particles barely interact with each other.

Lotty Ackerman and colleagues at Caltech 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% of the strength of the electromagnetic force and not conflict with any observations.

If the force is close to this strength, its effects might be detectable, as it should affect how dark matter clumps together.

"It might even help with some niggling problems we have now," says team member Sean Carroll. For example, it might explain why there are fewer dwarf galaxies than models predict.

Carroll even speculates that more complex dark matter might exist, forming dark matter atoms with their own chemistry - and maybe biology.