The pure size of the universe really blows my mind and nothing highlights in more than looking at images of galaxies.  We have barely sent objects outside of our solar system (Voyager I and II) in my lifetime and yet we are part of this massive galaxy called the Milky Way.  Then we realize we can see other galaxies, and each point of light is a star, probably with planets just like ours and some middle aged dude is using his telescope and taking a picture of our galaxy... mind boggling.