Arret was an inhabited planet in an unnamed star system. It was the homeworld for the Arret natives, a warp-capable humanoid species. This reverse negative antimatter universe planet was in the same galactic position as Earth in the forward positive matter universe.
Karla Five, her son Karl Four, and her father lived on this planet during the mid-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")