“There are actually blood vessels obstructing light from reaching certain areas in your eye, effectively creating a shadow. Your brain filters this out and essentially fills in the gaps so you don’t actually see this spiderweb-like network of black lines. However, you can visualise them by shining a light at a diagonal into your eye (not directly!) and gently wiggling it about. This means your brain doesn’t have enough time to filter it out and you see this spiderweb like network of blood vessels!’