Physicists Stephen Henrich and Keith Olive are breathing new life into a dark matter theory abandoned in the 1970s. Their "ultra-relativistic freeze-out" mechanism proposes that dark matter separated from ordinary matter much earlier than previously thought—during the reheating era right after cosmic inflation.

The original hot dark matter concept was rejected because fast-moving particles would have disrupted early galaxy formation. By moving this freeze-out event earlier in cosmic history, the particles would have had time to cool down, making them compatible with what we observe today.

This approach helps explain why decades of detection experiments have come up empty. Ultra-relativistic dark matter interacts even more weakly than WIMP candidates, sitting between WIMPs and FIMPs as a long-overlooked category that could finally solve the universe's missing mass mystery.