Learn Russian: В парке темно, и я никого не замечаю. - It is dark in the park, and I do not notice anyone.

Никого is the Accusative case form of никто (nobody/no one). Because "nobody" is the direct object receiving the action of the verb замечаю (I notice), it must be in the Accusative case. Remember that for animate nouns (people and animals), the Accusative case looks exactly like the Genitive case.

Why do we need both никого and не? Isn't that a double negative?

Yes, and Russian requires a double negative here! When you use a negative pronoun like никто (or its forms like никого), you must also put the negative particle не right before the verb. The literal translation of this part is "I nobody not notice."

Where are the words for "it" and "is" in В парке темно?

Russian doesn't use placeholder words like "it" or present-tense "to be" verbs like "is" to describe the environment or weather. The adverb темно (dark) acts on its own to mean "it is dark."

Why is the verb замечаю used here instead of its perfective partner?

Замечаю (I notice) is an imperfective verb, which is used here because it describes an ongoing state in the present. You are in the park right now, looking around, and in the process of not noticing anyone.