Here is something absolutely spectacular. When we look up at the night sky, we can see our own Milky Way Galaxy as this white fog like line all across it. It's a stunning sight. And we can also see distant stars, but we have no concept of other galaxies because their edges are not distinguishable by the human eye. They are not bright enough to be seen as a whole.
But what if we could see them? How big is our neighboring galaxy, Andromeda, really in our night sky? Below, is an image overlay of the night sky, with our moon, and the actual size of Andromeda.
Isn't this amazing?
It's huge! And keep in mind that it's currently 2.5 million light years away. Here is another picture from NASA: http://apod.nasa.gov/apod/ap130801.html
But, of course, it's going to get even bigger. Andromeda is on a direct collision course with our Milky Way Galaxy, currently approaching at a speed of about 396,000 km/h, and it will slam into us in about 4 billion years (although there is no real risk of actual planets hitting each other). And after a short dance of another 1-2 billion years, we will merge into a new larger galaxy. Here is a simulation https://www.youtube.com/watch?v=4disyKG7XtU
Of course, this makes little difference to us here on Earth. You see, Earth won't be a habitable planet by then. Our own sun is dying, and will slowly turn Earth into a wasteland over the next 3 billion years. So by the time Andromeda hits us, we will no longer be here.
Hopefully, we will have become a fully native spacefaring race by that time, with colonies on thousands of planets, as well as entire civilizations living in huge spaceships wandering the universe. http://goo.gl/iTBrr5