Think bigger than home, city, or state. Think bigger than country, continent, or planet. Think even bigger than solar system, galaxy, or supercluster. Only one entity encompasses all of these objects, and this is the universe. What's more, the universe is comprised not only of all that humans can directly observe, but also of everything that we cannot see. Along with the most familiar forms of visible matter, elusive entities such as quarks, neutrinos, black holes, and yet-undiscovered forms of dark matter compose the universe.

Simply put, the universe, or cosmos, as it is sometimes called, is everything. It is the entire body of objects and phenomena that may be observed or theoretically postulated. As the late Carl Sagan revealed with the memorable opening line of his bestselling book Cosmos, "The cosmos is all that is or ever was or ever will be."

Scientists continually take on the...