"Galaxy Knight" and "Galaxy-Eyes Photon Dragon", in the artwork of "Galaxy Expedition".
"Galaxy" (ギャラクシー Gyarakushī) is an archetype of cards used by Kite Tenjo, Nelson Andrews, Eviluder and Mizar in Yu-Gi-Oh! ZEXAL. The monsters tend to be quite powerful, while the Spell and Trap Cards usually function as support or have synergy with the related "Photon" archetype. The archetype is geared towards quickly Summoning the "Galaxy-Eyes" Xyz Monsters as fast as possible.
A "Galaxy" Deck focuses on Summoning several Level 8 monsters for Xyz Summons. By using "Accellight" to Special Summon a "Galaxy Wizard", you can change its Level to 8, and using "Galaxy Expedition" to bring out "Galaxy-Eyes Photon Dragon", you can instantly bring out a Rank 8 Xyz Monster. You can later use "Galaxy Zero" to revive a Level 8 monster, Normal Summon "Galaxy Knight", and revive "GEPD" to overlay for "Neo Galaxy-Eyes", "Tachyon Dragon" and "Prime Photon Dragon".
"Galaxy Wizard" can search out most of the important Spells and Monsters in the Deck, but using its effect leaves your Life Points exposed. Because of this, it's recommended to use "Photon Thrasher" to maintain field presence, with the added bonus that "Thrasher" can be used with "Wizard" for Rank 4 Xyz Monsters like "Starliege Paladynamo" and "Constellar Omega". Furthermore, Special Summoning "Thrasher" lets you Normal Summon "Galaxy Knight" without Tribute in the same turn. You can also use "Kuriphoton" to fend off OTKs, and return it to your hand by discarding a "GEPD" that you want to revive from the Graveyard.
The inclusion of "Galaxy Soldier" combined with "Galaxy Expedition" also gives a player the chance to incorporate some capable Rank 5 monsters, such as "Tiras, Keeper of Genesis", and "Constellar Pleiades".
An excellent combo is to discard a "Galaxy-Eyes" with "Galaxy Soldier" to Special Summon the latter, then use "Soldier's" searching effect to add a "Galaxy Knight" to the hand. "Galaxy Knight" can then be Normal Summon and revive the "Galaxy-Eyes" in the Graveyard, to bring out a Rank 8 monster.
Because this Deck is focused on multiple large Special Summons for high damage, your opponent may slow you down with "Torrential Tribute" or "Solemn Warning". It's a good idea to run "Starlight Road" and extra Spell/Trap removal.
Because the Deck will most likely be running many high-Level monsters, dead draws can be very frequent if your field or hand is not set up at the time to allow for their Summoning - this can happen quite easily if your field was cleared from an attack or card effect and you no longer have the resources to pull off the combos you rely on.