Star Trek: The Original Series
The Alternative Factor
S1 E27:
When the entire universe seems to "blink," the Enterprise suspects invasion but finds only Lazarus, a man who seems to alternate between two different extremes of nature. In time, "he" is found to be two twinned beings locked in eternal struggle between their matter and anti-matter universes — a rift Kirk must help seal.
Starring
Robert Brown, Janet MacLachlan, Richard Derr