Sterile neutrino

Sterile neutrinos, also known as neutral heavy leptons, heavy neutrinos, or right-handed neutrinos, are theoretical particles that are extremely difficult to find. This is because they are not affected by all of fundamental forces that rule normal physics, only gravity. They are detected by a theory called the seesaw mechanism which says that when sterile neutrinos are near other neutrinos (tiny particles which are not affected by much), they increase their own mass by lowering the mass of the nearby neutrinos. Detection probably comes from the fact that they are affected by gravity.
Think of it as a particle that absorbs the energy of other particles of the same family type. Once one is near another of its family, it raises its mass by lowering (absorbing) another's overall mass.