Title: Black and White/ Night and Day/A World Without Darkness

Genre: Adventure/Suspense/Action/Fantasty/ Alternate Universe

Plot:In this world, darkness is fluid while light literally up darkness

The world is constantly fighting against the encroaching darkness

MC decided to rid the world of darkness

Creates a huge light machine that purges darkness

After some years, world is finally free from the dark

No such thing as nighttime, nightmares or even shadows exist

Discord with night gone; no one can sleep without artificial darkness 

No one has seen the moon or a star in years

Hundreds of years pass, MC passes away and is reincarnated

Light has prevailed for a long time but darkness is slowly being established once again

MC tries to prevent darkness from regaining power again

Darkness is recreated and light and dark fight once again

MC dies with goal of world without dark unfulfilled

I swear I feel this kind of concept keeps popping into my mind.

Light versus dark. Light cannnot survive without darkness and vice versa.

Yep. Otherwise, moral of the story is you can't have darkness without light.

Pretty obvious.