This is hard. But I can attempt–have a look at the back of your hand, and pull out one of your hair. Ouch!! that is painful. But that is not the point. The point is one hair is almost like 1.0×10^5 atoms..you need a powerful electron microscope to see an atom
@Dolly da Gucci,
This is a challenging question whose answer I do not know. But is I remember any of my chemistry well, I think atoms differ in size as you move across the period. This size of an atom depends on the number of protons and the number of energy levels (shells). So I guess depending on what atoms the matter-or something- is made of the number of atoms needed for it to be visible will also vary.
Another possible explanation is that in order to see, the object should be able to release or reflect light. The ability to release this light is a factor of the energy that these atoms have so that you can have small number of high energy atoms making them visible-or a large number of very low energy atoms 🙁