The Opera House is a legendary structure said to have existed in the City of the Gods on Kobol, alleged in the Sacred Scrolls to be the birthplace of Humanity, along with other edifices such as the Forum and Temple.
These are the first ruins on Kobol that viewers see after Crashdown's Raptor, damaged by a Cylon Raider attack as they headed to explore the planet, crash-lands on Kobol near the Opera House. Among the surviving passengers on that craft were Galen Tyrol, Tarn, Seelix, Cally, Socinus, and Gaius Baltar.
Shortly after his Raptor crash on Kobol, the virtual Number Six leads Baltar towards the ruins of the Opera House, where viewers and Baltar see the Opera House as it was, millenia ago. It is in that virtual Opera House that Six shows Baltar an image of what she claims is their future child (Kobol's Last Gleaming, Part II).
Much later a Number Three would have similar visions of the Opera House and beings of light therein, who she believed to be the final five (Rapture).
In the vision of the opera house shown Baltar by Six, there are 8 white drapes. When Number Three sees her vision of it, there are 6 drapes, with 5 glowing white-robed figures standing on the rightmost 5 (from her vantage point.)
Notes
- The Orpheum Theatre in Vancouver (where Battlestar Galactica is produced) provides the setting of the Opera House interior that Gaius Baltar sees in his dream (Source: The Official Companion).