Someone who is dogmatic has arrogant attitudes based on unproved theories. If you dogmatically assert that the moon is made of green cheese, you'll just get laughed at.

The most basic definition of the adjective dogmatic is that it is related to dogma — doctrines relating to morals and faith — but what it has come to mean is attitudes that are not only based on unproved theories but are also arrogant in nature. The root of dogmatic is the Greek word dogmatikos. A synonym of dogmatic is "dictatorial" and because there are religious associations to the root word dogma, someone who is dogmatic tends to "pontificate."

Definitions of dogmatic

adj of or pertaining to or characteristic of a doctrine or code of beliefs accepted as authoritative

adj relating to or involving dogma

dogmatic writings”

adj characterized by assertion of unproved or unprovable principles

narrow, narrow-minded
lacking tolerance or flexibility or breadth of view