What do you call someone who thinks everyone is incompetent (doesn't know anything)?

The first word that comes to mind is arrogant.

People who actively interfere with other people's jobs - typically because they don't think they're doing a good job - are commonly called micro-managers.

In the political arena, I'd call such a person intelligent, as activists are typically astounded by the almost unbelievable ignorance of the masses, which they often refer to as sheeple.

However, I'm going off on a tangent; it sounds like that's not the context you're looking at.

I can't think of a good phrase for this, but if you are looking for descriptors, the two that come to mind are condescending and belittling.

Though these don't specifically incorporate this person's perception of the lack of knowledge of those he is belittling, at least they describe the resulting behavior.

What about pontifical? Google defines it as characterized by a pompous and superior air of infallibility.

"Having to explain every minute detail" is not explicit in the definition of the word, but it is the sort of behavior you might expect from someone who

patronizes: speak to or behave toward someone as if they are stupid or not important

other people.

That particular meaning works with the verb form (patronize) and adjective form (patronizing). Patronizer is out there, but it seems to get more use for denoting someone who engages in the more pleasant sort of patronizing (sponsoring, supporting, or bringing trade to). If you're willing to accept a phrase rather than a single word, you could describe someone as a "patronizing overexplainer."

If you're just looking for an adjective, another suitable word is

haughty: having or showing the insulting attitude of people who think that they are better, smarter, or more important than other people

though its definition also lacks the specific detail of feeling the need to explain minutiae to others.

P.S. I included links for the questioner because common search engines apparently proved too difficult. Positioning the mouse cursor over the red letters in italics and left-clicking with the mouse will open the pages with the quoted definitions.

Possibly related: a misanthrope is a person who basically hates the whole human race. In my experience, most misanthropes are misanthropes because they think that almost everyone is stupid. (That's my reason, anyway.)

Maybe you're looking for the Dunning-Kruger effect?

"The Dunning–Kruger effect is a cognitive bias wherein unskilled individuals suffer from illusory superiority, mistakenly assessing their ability to be much higher than is accurate. This bias is attributed to a metacognitive inability of the unskilled to recognize their ineptitude. Conversely, highly skilled individuals tend to underestimate their relative competence, erroneously assuming that tasks that are easy for them are also easy for others."

A know-it-all? Doesn't literally fit your definition, but does imply it, as someone who is convinced that they know everything and have all the answers is by extension likely to think that everyone else doesn't.

Well, if you twist it you can adjust your phrasing by saying "he acted superior towards his employees" or "being around this person gives me a false sense of inferiority"

But I believe the best word that comes to mind would be "Elitist", as it would also go along side with other words that describe its attitude towards other people and view on self.

Adjective

1. (of a person or class of persons) considered superior by others or by themselves, as in intellect, talent, power, wealth, or position in society:
   elitist country clubbers who have theirs and don't care about anybody else.

Dictionary.com

'Conceited' could be a good candidate for what you are looking for.

Someone who thinks they are so clever that they can do no wrong and everyone else does not measure up to their superiority.

floccinaucinihilipilification

(often humorous) The act or habit of describing or regarding something as unimportant, of having no value or being worthless.

I would call that person Laurence J. Peter, who originated the Peter Principle: "managers rise to the level of their incompetence." He studied managerial psychology and observed that all people are promoted until they reach a position at which they are incompetent, at which point they are no longer promoted.

In other words, I would call someone who thinks everyone is incompetent "Correct".

Pedant might apply. According to Wikipedia A pedant is a person who is excessively concerned with formalism, accuracy, and precision, or who makes an ostentatious and arrogant show of learning.

One adjective similar to many already mentioned, but which carries a sense of the tendency to administer to inferior intellects and to lecture, is

self-righteous 1. piously self-assured and smugly moralistic

(Wiktionary)

Looking into the definition, moralstic is defined as:

characteristic of or relating to a narrow-minded concern of the morals of others

(Wiktionary)

So as a descriptive term, self-righteous includes the notion that the person described, in being "piously," or intractably, "self-assured," most often would unjustifiably assume the inferiority of others' opinions and intellects. And in being "moralistic," the self-righteous person more often than not, whether out of a skewed notion of benevolence or just a desire to hurt someone's feelings, would attempt to correct the other's perceived faulty reasoning. This could extend to general criticism of of a person's competence, as opposed to restricting it to moral concerns.

There's contemptuous, from (of course) "contempt", which Merriam-Webster defines as "[the] feeling that someone or something is not worthy of any respect or approval".

I would call such a person smug. But it seems a bit of an understatement. Holier-than-thou is also a good way of putting it.

indignant

adjective
feeling or showing anger or annoyance at what is perceived as unfair treatment.

overweening (ˌəʊvəˈwiːnɪŋ Pronunciation for overweening )

adjective

1.(of a person) excessively arrogant or presumptuous

2.(of opinions, appetites, etc) excessive; immoderate

Superiority Complex: An exaggerated feeling of being superior to others. A psychological defense mechanism which allows an individual to overcome or conceal feelings of inferiority.

didactic (adj.):

1. (mainly disapproving) intended to teach, especially in a way that is too determined or eager, and often fixed and unwilling to change

'a didactic approach to teaching'

Source: CDO

2. (i) intended to teach, particularly in having moral instruction as an ulterior motive

   (ii) in the manner of a teacher, particularly so as to appear patronising

'his tone ranged from didactic to backslapping'

Source: ODO

He was very blank, where it would be completely clear that it includes the viewing other people as incompetent and going into extreme detail.

I'll offer a few ideas to choose from: Dismissive, superior, egotistical, domineering, disdainful, dismissive, narcissistic, asshole (sorry -- this one is not an adjective), dogmatic, supercilious, overbearing, having an inflated sense of his own worth/intelligency/abilities, alienated by his own inflated opinion of himself, disrespectful, belittling, too smart for his own good, hypercritical, snobbish, above everybody else, insufferable, and as someone already said, pedantic.

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Interfering, nitpicky. Incapable of delegating. This one isn't an adjective, but you could say that he breathes down everyone's neck. Backseat driver.

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Control freak. This means everything has to be done just so, and he can't allow anyone any autonomy or initiative. There is an adjective for this: controlling.

I'm reading your question...

I'm looking for a way to describe someone who thinks everyone is incompetent (i.e. doesn't know anything) and always has to explain every minute (small) detail because his perception of the person's competence is that it's very low (thinking the person is not that smart).

... and I'm thinking I know people like this. A boss I used to have and a close relative, in particular.

So to answer your question I began by asking myself what these two people had in common that might make them this way.

Some characteristics they shared (in my estimation) were:

• deeply insecure about themselves
• frequently compensated for their insecurity by convincing themselves (subconsciously, perhaps) that others can't possibly be as smart as they are
• their treatment of others was a defensive maneuver designed (again, probably subconsciously) to protect their fragile ego

The words that come to mind when thinking of these people include:

insecure, fragile, delusional, short-sighted and self-destructive

On a more superficial level (and maybe more relevant to the context of your question) I would say these people were:

inconsiderate, rude and condescending

The word pedantic can mean "ostentatious in one's learning," always showing off and lording it over others who are less knowledgeable.

Actually, "lording it over," might not be too bad.