The Force binds the galaxy together. ~ Obi-Wan Kenobi in the movie Star Wars (1977)
A galaxy is a cluster of star systems and stellar remnants, swirling in an interstellar mixture of gas, dust, and massive matter.
The structures in our present universe are the outcome of more than 10 billion years of evolution. Slight irregularities imprinted at very early eras led to increasing contrasts in the density from place to place, until overdense regions evolved into bound structures. ~ English astrophysicist Martin Rees
We do not know when galaxies emerged. The 1st galaxies we are aware of coalesced by 13.57 BYA. There were already mature galaxies with billions of stars 12.3 BYA.
(That galaxies existed only 250 million years after the supposed Big Bang strongly indicates that the standard cosmological model is bogus, as there is no astrophysical explanation for such rapid galactic evolution.)
The number of galaxies is much bigger than anyone would have guessed. And the real number could be even higher. ~ English astrophysicist Christopher Conselice
There are now ~4 trillion (4,000,000,000,000) galaxies in the universe; roughly half light and half dark. Each light galaxy may contain millions or even billions of stars. Almost all visible star systems have planets.
Within a few billion years of galactic formation, there were 10 times as many galaxies as there are today. Cosmic evolution reduced the number of galaxies through extensive merging.
To this day, mysterious filaments of galactic attraction thread the universe in an invisible gravitational web. Galaxies form along these filaments, with massive black holes as their hearts. The gravitational influence of filaments entices molecular hydrogen gas to coalesce, and so permits star formation. Galaxies run on gas.
Acting as a black backdrop to the glittering cosmopolitan cosmos, dark galaxies are conjectured to be as plentiful as the light variety. Some are utterly devoid of stars; others have a relative few. Black holes and gravitational filaments corral dense gas globules that shed no light. Little is known about the ecologies of galaxies, light or dark.