A number of short baseline neutrino experiments have found anomalous results throughout the past decades. These can be interpreted as oscillations between the active neutrinos and a sterile neutrino with a mass around $1$eV.
However, in the early Universe such a sterile neutrino would be thermalised through its oscillations with Standard Model neutrinos.
A fully thermalised sterile neutrino is incompatible with observations of the cosmological microwave background (CMB), large scale structures (LSS) and primordial abundances of the light elements leading to a tension between cosmology and the short baseline neutrino.
A new hidden interaction for the sterile neutrino mediated by a vector boson is capable of suppressing oscillations in the Early Universe and delay the production of sterile neutrinos until the Standard Model neutrinos have decoupled. This removes the tension with the light element abundances, but bounds on the neutrino mass from LSS and CMB observations still give tension with the model. Contrarily, a model with a pseudoscalar mediator coupled to the fourth neutrino mass state might relieve all the tension.