Dark matter direct detection is testing freeze-in

Dark matter (DM) may belong to a hidden sector (HS) that is only feebly interacting with the standard model (SM) and may have never been in thermal equilibrium in the early Universe. In this case, the observed abundance of dark matter particles could have built up through a process known as freeze-in. We show that, for the first time, direct detection experiments are testing this DM production mechanism. This applies to scenarios where SM and HS communicate through a light mediator of mass less than a few MeV. Through the exchange of such a light mediator, the very same feebly interacting massive particles can have self-interactions that are in the range required to address the small scale structure issues of collisionless cold DM.