John Wheeler proposed the idea that there was only one electron in the whole universe.
Positrons are the opposite of electrons, and are sometimes produced in radioactive decay or (rarely) in cosmic rays. They have one interesting property: you can model them like an electron that is travelling backwards in time.
The physicist John Wheeler (in a phone conversation with Nobel laureate Richard Feynman) proposed that this produced the unlikely possibility that all electrons and positrons in the universe are in fact be the same particle, just bouncing back and forth across time; in other words, it could be one thread interweaving with itself across time and space.
This would explain why electrons all look identical (they have the same mass and charge). It’s an elegant and poetic concept, and almost certainly not true.