There are many theories as to why new galaxies form. The newest theory is called the New Galaxy Formation Theory. Astrophysicists at the Hebrew University of Jerusalem have formulated a novel theory that takes issue with the prevailing view on the origin of galaxies. Their research, which was recently published in the journal Nature, concludes that the galaxies primarily formed as a result of intensive cosmic streams of cold gas (mostly hydrogen) and not as a result of galactic mergers. In fact, the scientists say that these mergers “had only limited influence on the cosmological makeup of the universe as we know it.”
The entire universe is constantly moving away, or expanding, from a single central point. The Big Bang theory explains that the universe was once in an extremely hot and dense state that expanded rapidly.
It was determined that space was expanding thanks in part to redshift, a shift in the spectrum that is linked to how far away an object is in our expanding universe. In 1929, Edwin Hubble discovered that the distances to far away galaxies were generally proportional to their redshifts. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.
There is a distinction between a redshift in cosmological context as compared to that witnessed when nearby objects exhibit a local Doppler-effect redshift. Rather than cosmological redshifts being a consequence of relative velocities, the photons instead increase in wavelength and redshift because of a feature of the spacetime through which they are traveling that causes space to expand.
Galaxies exist in the vacuum of space. When the universe is said to expand, the matter itself is expanding into the nothingness of space, rather than a destination. The vacuum itself is endless, or so vast that it might as well be considered endless. As we cannot see much of the universe due to our view being obscured by the gas and dust from our own galaxy, we have no real idea what lies beyond our own limited scope. Only time and advancing technology will tell.
Also extending far into deep space is the CfA2 Great Wall and the Sloan Great Wall. They are filaments of supercluster galaxies, and are some of the most expansive things in our known universe. It is not known how far the walls extend due to the plane of the Milky Way galaxy in which Earth is located. The gas and dust from the Milky Way (known as the Zone of Avoidance) obscure the view of astronomers and have so far made it impossible to determine if the wall ends or continues on further than they can currently observe.
Hope that helped!