Why is the sky dark at night?

from BoingBoing:

So, the sky looks blue because of the particular gases in our atmosphere reflect and scatter the blue wavelengths of light from the Sun. Fair enough. But that leads directly to a second question that, I'm ashamed to say, I never really thought to ask — why doesn't the light from all the stars in the Universe reflect and scatter off our atmosphere, producing a blue sky, all the time? This Minute Physics video provides a great explanation, which is grounded in both the timey-wimeyness of astrophysics and the limitations of our own human biology. Via BrainPickings