The maps of the US lights at night show that there is a line running down the middle of the country. East of that line it is essentially solid light, with little islands here or there of darkness (often large state or national parks). West of that line, it is opposite. Mostly dark, with islands of light at major cities.

What is puzzling is the location of the line. As far as I can see comparing maps, there is no obvious geographical feature that would have halted dense development at that line.

A good map showing this is the light pollution map at DarkSiteFinder.com: http://darksitefinder.com/maps/world.html

The line runs from Winnipeg in Canada, down through Grand Forks and Fargo, then Sioux Falls and Sioux City, then Omaha and Lincoln, then Wichita, then Oklahoma City, then down to Dallas and San Antonio.

I wonder if that line was just where westward expansion had reached before the Gold Rush and the rapid increase in migration to the far west, and afterwards people who would have otherwise went slightly west were instead going far west, and so we ended up with a sparse area in between?