As we know, the universe is an enormously huge place — "filled to the brim" with stars, galaxies, planets, and other interesting structures. Despite all this matter and material, quite a bit of space separates these various, far-off places. This holds true for most objects, except for certain structures that are gravitationally bound to another.
For example, our galaxy is just one of more than 70 in our local group. Our closest spiral neighbor, the Andromeda galaxy, is more than 2,500,000 light-years away. That may seem like a distant neighbor, but it's actually rather close. In about 5 billion years time, our galaxies will be within celestial spitting distance of each other, and eventually, we will merge to create an elliptical galaxy.
Here is another fascinating example of gravitationally interacting galaxies, known as Arp 194 (quite possibly the most beautiful one I've ever seen [including the Antennae]). This pair is located more than 600 million light-years from Earth (in the constellation of Ursa Major).
Generally, when the mergers begin taking place, that raw materials for star formation will be stretched out, forming a tidal tail of material that connects the interacting galaxies. This tidal tail is incredibly huge, extending more than 100,000 light-years across. The bright, oddly-shaped bits in the tail are indicative of star formation. This star formation kicks off when the material collects to the point that the clouds collapse under their own weight, sparking the start of the birthing process.
This particular region of Arp 194 is likely home to several star clusters also created through this process. Each contain many large, high-mass, blue-white stars. Also seen here are not two, but three active galactic nuclei, which means that there is a third galaxy involved in the merger (two of which can be seen in the northern portion of the image, whilst the other is pictured below). Eventually, all of them will coalesce to form a larger, more structured galaxy.
See a larger image here.