What on Earth, or off, is dark matter?

When astronauts look at Earth from space at night, they can see major clusters of city lights that identify urban areas like Paris, Chicago and Shanghai.

But the brightest areas are not necessarily the most heavily populated. Dark spots in the interior of China and India, are also teeming with activity.

Likewise, looking outward from Earth it's easy to assume that the dark spots in the heavens are empty. Looking into the vastness of the universe from here, humans since Galileo thought there was a lot of nothing out there.