From Bulbapedia, the community-driven Pokémon encyclopedia.
Team Galactic (Japanese: ギンガ団 Ginga Dan, Team Galaxy) is the villainous organisation in Sinnoh. Among other things, they try to entice Pokémon to do their bidding, attempt to invite the region's Pokémon professor to do research for them, and steal people's Pokémon.
Team Galactic wants to catch the Pokémon that created the world: Dialga of time and Palkia of Space, as well as Emrit, Agnome, and Yuxie so that their leader, Akagi, can destroy the universe and rebuild it in his own image with himself as an absolute God.
Admins and Boss
Also according to an issue of CoroCoro, all of the Admins of Team Galactic are named after planets:
Team Galactic's Japanese name, "Team Galaxy", is possibly a reference to the Galaxy Gang in Pulseman.