It is sometimes hard to grasp just how incredibly large a galaxy is, much less the entire Universe. An average galaxy can contain over a billion individual stars, and most galaxies are constantly creating new stars which coalesce from the massive clouds of interstellar dust. As far as current estimates go, there are likely over a trillion galaxies in our observable universe. This is an amazing amount, and the human brain does not have the ability to fully comprehend such large numbers.
Galaxies have life cycles too. Even though the distances between stars is vast, the distances between individual galaxies are exponentially larger. It is rare to see galactic collisions, but the Hubble Space Telescope has found many. Our own Milky Way galaxy is rushing towards the Virgo Galactic cluster, and in a billion years or so will be actively "colliding" with the cluster. This is what is happening in the image above. (Click image above to enlarge.)
The Hubble Space Telescope has captured the above image of three different galaxies in the middle of a collision. Even though scientists term this a collision, it is very rare indeed that any two individual stars would actually crash into each other, the distances between them being so vast. It is like having two people blow smoke at each other. The smoke particles would intermingle and affect each other but would likely never actually touch. In galactic collisions the same is true. However, the gravitational pull of all the stars and the galaxies collective gravitational power do affect each other. For example,
The three pictured galaxies — NGC 7173 (middle left), NCG 7174 (middle right) and NGC 7176 (lower right) — are part of the Hickson Compact Group 90, named after astronomer Paul Hickson, who first catalogued these small clusters of galaxies in the 1980s. NGC 7173 and NGC 7176 appear to be smooth, normal elliptical galaxies without much gas and dust.
In stark contrast, NGC 7174 is a mangled spiral galaxy, barely clinging to independent existence as it is ripped apart by its close neighbours. The strong tidal interaction surging through the galaxies has dragged a significant number of stars away from their home galaxies. These stars are now spread out, forming a tenuous luminous component in the galaxy group. - ( Science Daily )
The gravity of the large elliptical galaxies is effectively ripping apart the barred spiral galaxy. Scientists expect this process, which will take hundreds of thousands, if not millions of years, to create a massive galaxy, tens of times bigger than our own Milky Way. If only we could create a time-lapse camera that could capture the entire process. It would look something like this.