Does it seem impossible? Well, it is. Truth be told, there are no black dots. Each time you focus your eyes, you see that the dots you're looking directly at are all actually white. The surrounding ones just appear to be black.
Your brain's vision system naturally processes edges of objects so they are enhanced. Seeing edges clearly is important for your brain to understand and define an image. This can sometimes result in visual "artifacts" away from the center of the receptive field, such as the black dots in the Hermann grid above.
