Karl Four on the planet Arret
Arret was an inhabited planet in the reverse negative antimatter universe. It was located in the same position as Earth in the Milky Way Galaxy in the positive matter universe, but on a different dimensional plane.
Karla Five and her son Karl Four lived on this planet during the 23rd century. The inhabitants of the planet began their life at an old age and slowly grew younger. Brain functions, language and operational controls worked in reverse of what they did in the positive matter universe, as the flow of time was reversed.
In 2270, while transporting Commodore Robert April to his retirement ceremony on Babel, USS Enterprise accidentally visited the antimatter universe. (TAS: "The Counter-Clock Incident")
Note that "Arret" is "Terra" spelled backwards.