It’s an amazing optical illusion… When you look at a black dot, the others disappeared!
How many black dots can you see?
The optical illusion that you are looking at, is called Ninio’s extinction illusion.
When the white disks in a scintillating grid are reduced in size, and outlined in black, they tend to disappear. One sees only a few of them at a time, in clusters which move erratically on the page. Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are not actually present. Some black sparkling can be seen at those crossings where no disk is seen. The illusion also works in reverse contrast.
source Will Kerslake