A galaxy is a dense grouping of stars, held together by powerful gravitational attraction. Because of the extreme distance between galaxies, most of the lifeforms known to science are in our home galaxy, the Milky Way. There has been limited contact with objects, forces and life from outside this galaxy, however.
Milky Way Galaxy
The Milky Way Galaxy is a large barred spiral galaxy that is approximately 100,000 light years in diameter, and contains over 400 billion stars. The galaxy is composed of three major parts: the core, the disc, which is the ring of stars and interstellar dust that gives the galaxy its spiral shape, and the halo, which includes many older stars orbiting the core, but outside the disc, of which most are concentrated in massive globular clusters.
The Milky Way's disc is surrounded by a massive energy field of negative energy called the Galactic barrier, which makes travel into and out of the galaxy difficult.
In 2269, the center of the galaxy was explored and found to be a creation point. By 2287, the core had become surrounded by the Great Barrier, containing a planet thought to be Sha Ka Ree by Sybok.
In the science of stellar cartography, the Milky Way is divided into four major areas called quadrants: Alpha, Beta, Gamma and Delta, each of which compose one-quarter of the galaxy.
Between 2064 and 2364, Humans had charted 11% of the galaxy, Within a year, the Federation had charted an additional 8% of the galaxy.
Only one out of every 43,000 planets in the galaxy supports intelligent lifeforms.
According to Dr. Leonard McCoy, there is a mathematical probability of three million Class M planets in the galaxy.
An expedition sent by the Kelvan Empire, from the radiation-imperiled Andromeda Galaxy, scouted the Milky Way for possible invasion in 2268.
See the original version at Memory Alpha