Darkness in the primordial human mind represented the great unknown & in some cases, foreboding. In primordial, ancient, & medieval times, darkness represented the unseen & unknown. Darkness represented forces which were deemed suspect, even evil. It was no accident that primordial, ancient, & medieval people associated darkness with things evil & nefarious. Such people associated demons & other evil spirits/entities w/night time.
Night time, in their minds, was the most fearful time of the day. Night time was the unknown as one didn't know what will occur. It was also thought that night time was associated w/things that were undercover & oftentimes evil. In the primordial human mind, witches, vampires, ghosts,, phantoms, & even devils were night creatures.
Such ideas & associations aren't only associated w/our more primordial ancestors but subconsciously are planted in the modern mind. Darkness is still viewed as suspect, even evil. It is no mistake that horror movies have a theme of darkness. Many times, night time is the prevailing theme of horror movies. Night time adds that authenticity to horror movies which make such movies more believable to the human eye.
Darkness also is the time when nefarious activities occur. Criminals & other unsavory elements are the most active at night because they can indulge in such activities w/o fear of getting caught. People are oftentimes warned by family & law enforcement authorities not to be alone at night because at that time, they will be subject to such unsavory elements.
There are other people who contend that demons, devils, & other evil entities are most active at night. They see the night as a time of great trepidation because they believe that these spirits are present & are out to do them harm. A subset of such is the fear of the so-called witching hour which is THE DARKEST time of night. There are people who believe that 3 am at night is the so-called hour of the dead or the devil's hour. The night (darkness) is when the dead & the devil are at their most potent.