WIMPS. The more massive members of each lepton pair are the electrically charged electron (the outer constituent of atoms) and two other particles that resemble electrons in all measurable ways except that they are heavier, and are unstable in today's modern universe.
These pairs are difficult to break up, so any intruding electron, unable to displace an incumbent from a pair, must occupy a higher, vacant energy level within the superconductor in order to stay put.