Magratheans are the inhabitants of the legendary planet, Magrathea. Because their planet-building operation was so successful, they reduced the rest of the galaxy to abject poverty.
Since no planet was rich enough to buy a planet from them, all Magratheans decided to sleep through the recession until the galactic economy returned to a level which could require and pay for their services. The only individual Magrathean to appear in the series is Slartibartfast.