It's hard to fathom the idea of galaxies meeting, especially if you don't really understand what a galaxy is. So before we dive into the concept of how galaxies collide, let's first take a look at what galaxies are.

Galaxies are large systems of stars, gas, dust and dark matter. More than just free-form blobs, galaxies are held together by gravity. If you point a seriously powerful telescope into the sky, you'll see that galaxies come in different shapes and sizes. The shapes of galaxies were first classified by Edwin Hubble, the American astronomer credited with proving the existence of galaxies beyond our own. Hubble's research showed that galaxies typically come in elliptical, spiral and so-called "irregular" shapes.

Instead of floating randomly around in space, galaxies actually exist in groups called galactic clusters. Our galaxy, for example, is just one within a cluster called the Local Group, which also contains our nearest neighbor, the Andromeda Galaxy. Within each cluster, galaxies are constantly moving. In fact, thanks to Hubble's research, we know that the expansion of the universe causes galaxies to move farther and farther away from each other over time. But since galaxies are also constantly moving along trajectories in space, they sometimes meet and collide.

When galaxies collide, it isn't exactly like watching a fender-bender on the highway. You'd expect them to smack into each other and either crush one another or explode -- like cars do in a collision. But that doesn't happen. When galaxies collide, they actually pass through one another, and two things happen: The collision distorts both galaxies' shapes, and the massive amount of colliding matter forms new stars and black holes.

It's rare to witness galaxies colliding. For every million galaxies we can see at any given time, we observe about one collision [source: NASA, ESA and Estacion]. Still, scientists estimate that about 50 percent of galaxies have been involved in a collision at some point in time [source: Freudenrich]. We can tell by looking at their shapes: Collisions between spiral galaxies seem to leave behind elliptical galaxies. We also know that in about 5 billion years, our own galaxy -- the Milky Way -- is going to collide with the Andromeda Galaxy to form a new mega-galaxy. If humans are around to see it, the meeting of these two galaxies will leave our neighborhood of the universe almost unrecognizable.

(NASA)

Galaxies move through the universe and can even collide. Scientists estimate that 50 percent of galaxies may have had collisions of some sort with other galaxies. A galaxy can pass through another galaxy and come out the other side. The distance between the stars within the galaxies is so great that they don't actually strike each other. The shape of the galaxies, though, may become distorted by this movement. A spiral galaxy may become an elliptical galaxy.