Might As Well Be Infinity...

To expand upon this post a bit, Voyager I is almost at the edge of our galaxy. To put that in perspective, TPM writes, it's "some 11 billion miles away from the Sun, with its signal taking about 17 hours to get back to Earth." It took Voyager - a nuclear-powered spacecraft - 35 years to travel to the edge of the solar system.

It's always hard to really grasp distances like this, but Alan Lightman devotes an entire article ("Our Place in the Universe") to it in the December 2012 issue of Harper's:

"...Newton correctly concluded that the nearest stars [beyond our solar system] are about 100,000 times the distance from the Earth to the sun, about 10 trillion miles away. ... If we traveled in the fastest rocket ship ever manufactured on earth, the trip would last 100,000 years, at least a thousand human life spans." 

Pretty stunning numbers. And that's just to our nearest star, to say nothing of other galaxies. Lightman quotes a scientist studying a galaxy named UDFj-39546284 that is 100,000,000 trillion miles from earth. Damn.

Practically, this means that, since our current understanding is that speed-of-light travel is impossible, the rest of the universe is out of reach for manned space travel. Its reasons like this that make SciFi authors like Kim Stanley Robinson limit themselves to our solar system: the distances between systems are just too vast. And they're getting bigger all the time!