Question: What is the black in space?

Susan Cartwright answered on 14 Jun 2015:

The black in space is mostly just the absence of (visible) light. During the day, the sky here on Earth is bright because the atmosphere scatters sunlight: the blue of a clear sky, or the white or grey of clouds, is just light from the Sun that’s been bounced about so that it isn’t coming from the Sun’s direction anymore. In space, there isn’t very much material to bounce the light about – space is nearly a true vacuum – so if there isn’t a star in that particular direction, you don’t see any light. (In fact, if you point a telescope at a piece of “black” sky, you will see lots of luminous objects – see, for example, the Hubble Deep Field and Hubble Ultra Deep Field – but they are too faint for your eyes to pick up.)

This accounts for most of the “black in space”, but there are some regions which appear black despite the fact that there is clearly some background light. One of the most famous of these is the Horsehead Nebula in the constellation Orion. These “dark nebulae” are dark because they are dense clouds of cool gas and dust, cool enough not to emit visible light themselves, and dense enough to block light from behind them.

It is important to realise that the “black in space” is black only in visible light. If you were to look at some other wavelength, you would not see black. For example, the Horsehead is a bright object at infra-red wavelengths (longer than visible) – it is too cool to emit visible light, but warm enough to emit infra-red. And the whole sky is bright at microwave wavelengths (about 1 mm), because of left-over background radiation from the Big Bang.