Here on Earth, there are enormous variations in the densities of what we commonly encounter. Solid, dense metals, like gold or tungsten, have very high densities. If you had a cube that was one meter (3 feet and 3.39 inches) on a side — a cubic meter — made of gold, it would weigh approximately 19 metric tonnes: over 42,000 pounds. If that cube were instead made of water, it would weigh only 1 metric tonne, or 2205 pounds. Make that same cube out of air at room temperature and at sea level, and it weighs in at just around 1.2 kilograms, or 2.6 pounds. And even though it’s a struggle to create a vacuum on Earth, a volume of space as devoid of particles as possible, we’ve created apparatuses that reduce densities to less than one-trillionth of the air density normally found on Earth. But we often talk about outer space as being the ultimate in emptiness.…