This is the Hermann grid illusion. It’s a very well-known and
simple optical illusion. When you look at the intersections of
the white lines, you see illusory dark blobs. The standard
explanation for this has to do with retinal ganglion cells.
These cells pool information from several rod and cone cells in
the retina and transmit it to the brain. The area each ganglion
cell collects information from is called its receptive field. In
the middle of each receptive field, light stimulates the
ganglion cell, causing it to fire more frequently. However, on
the periphery, light has a paradoxical opposite effect: the more
light hits the peripheral receptive field, the less the ganglion
cell fires. The theory goes that, at the intersections, more of
the peripheral receptive field is stimulated than elsewhere, and
so the ganglion cell is inhibited, and the area appears darker.
Here’s an illustration of this: This would also explain why,
when looking directly at the intersections up close, you don’t
see the dark blobs,