I googled antonym of optimistic and the results are gloomy, negative, defeatist, downbeat, gloom-ridden, cynical, bleak, fatalistic, dark, black,despairing, despondent, depressed, dejected,demoralized, hopeless, melancholy, glum,lugubrious, suspicious, distrustful, doubting, and alarmist. I am looking for a common and proper antonym of optimistic, which describes human. Is pessimistic a big word or a comman in colloqiial conversatuon? Also, dark seems refering to evil such as dark side and dark force in Star Wars the movie. Is it really a proper antonym of optimistic?
The (most) direct antonym of optimistic is pessimistic which can be used both to describe a person or something more abstract.
Is it a "big" word? Well, no bigger than optimistic. According to online etymology dictionary they originate from the same period (optimistic from 1845, pessimistic from 1866) and they came into English in the same way. Optimistic is from the word optimist, which came to English from French, and optimist is from optimism which originally came from Latin, but also via French. Etymoonline suggests the same path of origin for pessimistic: pessimism -> pessimist -> pessimistic.
This google Ngram suggests that optimistic is used somewhat more often than pessimistic, but these are two opposite words so I think that frequencies of usage are not directly comparable. On the other hand, pessimistic is used more often than defeatist e.g., which is one of the proposed antonyms for optimistic on your list.
Optimistic and pessimistic can be used in the same type of writing or speech, but of course with the opposite meaning. If you want something more colloquial you could try negative although it might have a slightly different meaning, and is a direct antonym of positive.
As for the word dark in the meaning "deeply pessimistic" it is more suitable for abstract nouns than for people:
a dark vision of the future
A folky lament on death and love, it never sounds as dark as its lyrics intend because of tremendous harmonies.