Planet of Origin: Magrathea, located in orbit around the twin suns Soulianis and Rahm in the heart of the Horsehead Nebula.
Magratheans are an advanced humanoid alien species which, during ancient times of the Galactic Empire, was known as the famed planet-builders; Hyperspatial engineers sucking matter through white holes in space to form dream planets as their advanced technology and artistic skills allowed them for other species according to the specifications of their clients. Ultimately, such a tremendously expensive and lucrative business accidentally resulted in the galactic economy being ruined and so the Magratheans went dormant for billions of years, waiting for it to recover. Meanwhile, their species, planet and history were slowly relegated to the status of mere legends.
Among the planets built by Magratheans was Earth; actually a supreme computer with an organic matrix requested by the Pan-Dimensionnal Mice, who used it to search for the ultimate question about life, the universe and everything else (the answer to which they already know to be 42).
Source: The Hitchhiker's Guide to The Galaxy