There is only one file type that uses the .GALAXY extension:
Blizzard Galaxy File
Source code file created using Blizzard's Galaxy language, which is used to program game triggers and computer artificial intelligence modules for StarCraft II: Wings of Liberty; contains variables, instructions, and other logic for the game; used for controlling the way the computer plays against human players, or for scripting game triggers.
GALAXY files are typically included in Blizzard Mo'PaQ archives, which use the .MPQ extension. They can be edited with the StarCraft II Editor (also referred to as the "Galaxy Editor"), which is included with the StarCraft II installation. Other files that include GALAXY files are .SC2DATA and .SC2ARCHIVE archives, both of which store data using the MPQ format.
Updated: December 19, 2010