13,000,000,000 BBY

"This is our galaxy. Its luminous disk is about a hundred thousand light years across and contains some four hundred billion stars&mdash;only about a quarter of which have been thoroughly surveyed."

- Crix Q5 Baobab, teaching at the Baobab Merchant Marine Academy's first day of orientation

13,000,000,000 Before the Battle of Yavin, also 12,999,963,537 Before the Tho Yor Arrival, 12,999,996,347 Before the Treaty of Coruscant, or 12,999,999,965 Before the Great ReSynchronization, was the approximate year, firmly in the Pre-Republic era, in which it was believed that the galaxy was formed from universal debris. Over the ensuing eons, star systems and planets formed within the galaxy, with life and sentience growing upon these worlds' surfaces, leading to the events of galactic history.

The galaxy in 13,000,000,000 BBY
By the year 13,000,000,000 BBY, long before the Galactic Republic was founded, the Big Bang had already taken place, creating the universe. Many believed that it was approximately the year in which a cloud of dust and gas within the universe that was one-hundred-thousand light years across experienced a gravitic collapse; the cloud coalesced to form a disk of the same dimension that revolved around an extremely massive black hole, the galaxy.

Impact
"What's that? Oh, you want an answer to the life question. Well, nobody knows for sure&mdash;remember only about a quarter of the galaxy has been properly surveyed. But there are more than five million intelligent species known to the Empire. Do the math and you'll realize there have to be a fair number of species we've yet to meet."

- Crix Q5 Baobab, teaching at the Baobab Merchant Marine Academy's first day of orientation

The galactic disk came to have three vertical layers: one in the middle that was 2,000 light years thick called the "thin slice" sandwiched between two 3,500 light year sections on either side of that called the "thick slice." The majority of the galaxy's approximately four-hundred-billion stars fell within the thin slice, though the thick slice had some and there was also a great sphere around the galactic disk called the "stellar halo" that consisted of billions of stars with highly elliptical orbits. Many of the stars ended up developing planets that orbited them over the succeeding eons. Half of these star systems &mdash;primarily within the thin slice &mdash;maintained conditions favorable to life, ten percent of this half actually developed life, and only one in a thousand of the life-bearing worlds went on to bear true sentience. Life would still be emerging on planets in the galaxy by 5,000,000,000 BBY. Many of these forms of sentience would try to explain their conditions, forming religions. A number developed creation myths, detailing how their existence came to be,  whether that meant the creation of the galaxy, the formation of their world, or the origin of life. The Mandalorian religion had a creation tale called the Akaanati'kar'oya, "The War of Life and Death" in Basic, and it posited that the stars in the sky were ancient Mandalorian leaders that had died. Life that grew on worlds separated by great distances, primarily twenty million sentient species, would go on to interact and integrate, experiencing and eventually recording galactic history.

By the reign of the Galactic Empire, which began in 19 BBY, much would be known about the physical properties of the galaxy, from its stars to its planets, but only about a quarter of all stars in the galaxy were surveyed. Contemporary estimates stated that of all the stars, 7.1 billion were not dangerous to life while 3.2 billion of those supported actual star systems&mdash;it was proposed that at least one billion of them were inhabited to some degree and Imperial census data showed 69 million that were populated significantly. There were 1.75 million planets considered by the Empire to be advanced and populous enough for full member status. Imperial records further showed five million cataloged examples of sentient species, but it was accepted plainly that there were many more undiscovered species in the galaxy.

Behind the scenes
Initially, 5,000,000,000 BBY was the approximate date of the creation of the galaxy, as established by a timeline in The Essential Guide to Alien Species (2001) by Ann Margaret Lewis. The Essential Atlas (2009) by Jason Fry and Daniel Wallace denoted the year 13,000,000,000 BBY as a modern date, and this timeline alteration rectified a potential inconsistency. Before Lewis's Essential Guide was released, Goroth: Slave of the Empire (1995) by Nigel D. Findley placed 5,000,000,000 BBY as the year around which life first emerged on Goroth Prime&mdash;a planet which would have taken quite a long time to form, let alone develop life.