Andromeda (also known as M31, Messier 31 or NGC 224) is the closest spiral galaxy to the Milky Way at 2.5 million light years. It is located in the constellation Andromeda. It is a type Sb galaxy, giving it spiral arms similar to the Milky Way, however, it doesn't show a barred shape through its center.
Andromeda, to Human knowledge, was first thought to be a nebula (as the Great Andromeda Nebula), before Edwin Hubble discovered galaxies and their motions in 1929. When Edwin Hubble observed Andromeda and other distant objects, he realized that they weren't nebulae, they were cities of billions of stars.
Since Humans begin intergalactic exploration, they realized Andromeda and billions of other galaxies were home to thousands of alien, starfaring civilizations.
During the First Galactic War, signals were sent out into intergalactic space, many of them reached the Andromeda galaxy. However, many of the alien civilizations in Andromeda didn't interfere with the war.
Upon the arrival of the New Order in 8410, they soon achieved the status of galactic rulers in 8463 and held that status for 98 years until 8561. In 8583, the new female leader Jeira Lyn ordered the construction of a gigantic fleet and sent it towards the Milky Way in 8589.
In 121100 CE, a newly formed government known as the Socialist Planets of Taitheia was formed around Andromeda's core worlds. Jointly run by humans and Taitheia, this interstellar government has lasted for millenia and has now become a member of the Confederacy of Humanity.
Andromeda is believed to be approximately 9 billion years old, forming when the Universe was only 4 billion years old. It gets its spiral shape from its rotation. One rotation of the galaxy can take hundreds of millions of years to complete. Andromeda is about 141,519 light years in diameter. It is believed 1 trillion stars orbit within the galactic disk. Among these stars are thousands of alien starfaring civilizations, older or younger than Humans.
Collision with the Milky Way
In about 3 - 4 billion years into the future, the Milky Way will meet the Andromeda galaxy in a galactic collision. The collision between both galaxies will rip each others spiral arms apart. Eventually, the two galaxies form the new Milkomeda galaxy. It will most likely become an elliptical galaxy. Although star formation is slowing down in both galaxies, the collision may trigger the formation of millions of new stars and solar systems.