The Stellar Galleries are situated in the High Council Pavilions of Iacon. They are housed in a massive, faceted, breathtaking dome with multiple other facilities, among which is a map room containing a giant statue of Cybertron with small planets around it. All together, it serves as a center of contemplation and planning for Iacon's greatest thinkers.
The Omega Key was kept in a container in the Stellar Galleries, so when Megatron needed it to access the core of Cybertron, he ordered an assault on Iacon. Though he had Starscream lead a distraction assault elsewhere, the Autobots weren't foolish enough to leave the Stellar Galleries unprotected and fought to keep him and his soldiers at bay. They failed, as did the security of the Stellar Galleries. By infusing the doors and control panels with Dark Energon, the Decepticons reached the room holding the Omega Key with ease.
It was too early to declare victory though. The container turned out to be empty, and a hologram of Zeta Prime appeared to the Decepticons to tell them the Omega Key was with him now and that they got one more chance to withdraw. Megatron mocked him, declaring he'd get the Omega Key from his corpse then, and was promptly attacked by an Autobot unit. Once those opponents were defeated, Megatron ordered Soundwave to trace Zeta Prime's broadcast signal, which revealed they could find him at the Iacon Vaults.