Seriously, just stare at this picture for some 10-20 seconds – blinking is ok; you’ll see it vanish, bit by bit.
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.
So if you fixate on a certain point, after approximately 20 seconds, stimuli which fit in your peripheral vision will fade away and disappear. The effect is most powerful for some colours and patterns, and is more powerful the farther the object is from your center of fixation.
Here is another example of the phenomenon: the spots in the “lilac chaser” illusion fade away after several seconds when the black cross is stared at long enough.
