If stare at this image for 20-30 seconds it wil completely disappear, but you must keep your gaze completely still and not blink or move your eyes. I tried it and it works.

This is called the Troxler effect, after the man who discovered it: Ignaz Paul Vital Troxler, in 1804.
What happens here is that your visual system constantly adapts to all the external stimuli. This is why after spending a few moments in the dark, you start to see a little better. This capability allows you to be in different lighting conditions, while still maintaining a pretty accurate estimate of the lightness and colour of objects.