Imagine you had a spacecraft capable of travelling through interstellar space. You climb in, blast into orbit, fly out of the solar system and keep going. Where do you end up, and what do you see along the way? The answer is: mostly nothing. Space is astonishingly, mind-blowingly empty. As you travel through the void between galaxies your spaceship encounters nothing more exciting than the odd hydrogen molecule. But when it does come across something more exotic: wow!