It's too dark to know how far down they go.

CNN: workers

"It's much too dark in this little dive to continue slogging my way through IQ84!"

CNN: How to kill time without your phone

This will allow the virtual display to show things such as weather and other aeroplanes, further increasing the pilot's awareness when it is too foggy or dark to see outside.

ECONOMIST: MONITOR

We think dark matter is a particle too, it's just an odd form of matter in the fact that we don't perceive it very readily.

BBC: Dark Matter: Experiment to shed light on dark particles