It may look like a moving GIF, but it's actually just Ninio’s extinction illusion. The grid features 12 black dots, however, it’s nearly impossible to see them all at one time, unless you want a headache.
The illusion was developed by Jacques Ninio, a biologist who specializes in visual perception, who adapted the famous “Hermann grid illusion". Game developer Will Kerslake shared it on Twitter on Sunday and it quickly made its way to the top of Reddit.
As Ninio explains in a scientific paper about the 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.”
Speaking to The Verge, vision scientist Derek Arnold explains how this illusion is all to do with our relatively poor peripheral vision. Essentially, our brain attempts to account for this uncertain periphery by simply following the regular pattern of gray lines.