What is it called when experts think they only know a small part of a topic and amateurs think they know almost all of a topic?

What is it called when experts think they only know a small part of a topic and amateurs think they know almost all of a topic?

Solution 1:

Sounds like the Dunning–Kruger effect:

The Dunning–Kruger effect is a cognitive bias in which low-ability individuals suffer from illusory superiority, mistakenly assessing their ability as much higher than it really is. Psychologists David Dunning and Justin Kruger attributed this bias to a metacognitive incapacity, on the part of those with low ability, to recognize their ineptitude and evaluate their competence accurately. Their research also suggests corollaries: high-ability individuals may underestimate their relative competence and may erroneously assume that tasks which are easy for them are also easy for others.—Wikipedia: Dunning–Kruger effect.

Solution 2:

when experts think they only know a small part of a topic

This sounds similar to Impostor Syndrome.

and amateurs think they know almost all of a topic?

As Laurel already mentioned, this is called the Dunning-Kruger effect.