To get a sense of how big the universe is, and all that it contains, scientists usually speak in terms of galaxies. By recent count, there are likely as many as 100 billion galaxies, each containing billions upon billions of stars. The nearest galaxies to our own Milky Way are over 180,000 light-years from us. They were actually discovered by world explorer Ferdinand Magellan in a 1519 voyage—that’s before the invention of the telescope! We can think of these galaxies as cities at night view from the air: bright spots amidst the dark of surrounding countryside. As with the dark of the countryside, however, there is actually a lot going on in the void between these celestial clusters of stars and planets. Some of the most important and interesting discussions among astrophysicists deal with these spaces between the galaxies.

Thanks to modern technology and recent theories, we have been able to detect the presence of objects and phenomena never before possible: dwarf galaxies, exploding runaway stars, dark matter, and dark energy—to name a few.