Magratheans are a humanoid alien species native to the legendary planet of Magrathea. During ancient times of the Galactic Empire, the Magrathean race was known galaxy-wide as the famed planet-builders; as their advanced technology and artistic skills allowed them to build entire planets 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-Dimensional 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).