Exactly. The only thing that is really certain is that correction terms are required when fitting models to observations and these take on the role of mass and/

Exactly. The only thing that is really certain is that correction terms are required when fitting models to observations and these take on the role of mass and/or energy. The usual working assumption is that if a well established theory indicates the existence of extra mass or energy, it usually exists in nature and can be found. This is essentially something that has its roots in particle physics: every violation of conservation of energy or momentum turned out to be caused by a hitherto unknown particle.