The Real Reason It's So Dark In Space

Deep space is immense, brimming with potential — and not unlike Henry Ford's Model T, you can get in any color, so long as that color is black. 

Cast your gaze upwards during the daytime and you'll see an azure field of pristine blue and, maybe, a biplane dragging an ad for fried chicken. Tilt your head back 90 degrees in the evening, and you will be met by a dizzying reminder of the cold, terrifying emptiness that awaits, just a few miles off the ground. But hey, on the up side, there are stars. Lots of stars. Not as many as you'd think, really, given the potentially boundless nature of the cosmos. Come to think of it, why aren't there more of them? In a such a big universe, why isn't every corner of the night sky pockmarked by the possibility of turning Pinnochio into a real boy? Basically, why doesn't outer space look like the Lite-Brite of an obsessive completionist?

Why is empty space so dark?

It turns out that space is inky and black because — and apologies for any forthcoming existential crises — it isn't infinitely large or old. It's limited. It had a start date, and printed somewhere on the underside of a far-off nebula, or some solid stardust, it just as definitively has a sell-by.

The understanding that an immeasurably large universe, stuck in one place, would result in a lightboard night sky? That's called Olbers' Paradox, according to NASA, named after the 19th century German astronomer Heinrich Olbers. It's sort of the godfather of the Big Bang theory. Since space is dark, we can surmise that the universe is neither static nor infinite: entire galaxies are moving away from us, and they've only been able to get so far in the limited time that they've been around. There are a few other factors in the mix. Special relativity plays a small role, according to the UC Riverside Mathematics Department, and redshift also contributes — I.E., light can change color when travelling through gravity wells or moving across enormous distances, while the light source is in motion.

See? Astronomy is easy.