“ Within scientific disciplines, discovery is the observation of new phenomena, actions, or events which help explain the knowledge gathered through previously acquired scientific evidence.”
Straight from wikipedia (page on discovery).
“ In science, the term "theory" refers to "a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment." Straight from wikipedia page on Theory.
So if something is theorised to exist, it is neither a theory or discovery. Both cases need evidence and this has only now been presented.
Within science, someone can not “realize” something exist and claim discovery.