The Foo are a bunch of nebulous cosmically-powered aliens whose experiments played a major role in shaping evolution throughout the Looniverse. Supposedly. They claimed credit for seeding Earth with life.
They terraformed part of Mars and attacked Loonivearth with an army of drone ships and their servitors Cosmic Himbo and Space Goth GF. The Legion of Net.Heroes defeated them when Catalyst Lass convinced their servitors to switch sides and Nit-Pick Lad pointed out how their claim to have created life on Earth didn't fit with the existence of the RACelestials and the Xinerama Brotherhood, leading them to decide this part of the cosmos was too silly to bother with.
The Foo are based on Builders from Jonathan Hickman's Avengers run. They were named after the metasyntactic variable "foo" because the author of the issue that introduced them wanted to get it done quickly.