Is it true that in ancient times many thinkers identified light with existence (essence), and darkness with non-existence?
Examples include Pythagoreanism, Neoplatonism, the ontology of light as-Suhrawardi.
Absolutely. Light as existence and being are intertwined conceptually on a symbolic level especially in the philosophy of art, mythology, and religion as a common theme easy to see through the eye of philosophy of religion. God is purported to have uttered "Let there be light!" in the Old Testament, and it's still a contemporary idiom expressed during the act of creation. And Light and Darkness in Ancient Greek philosophy and religion is a complex topic (brynmawr.edu).
Zoroastrianism interpreted Darkness and Light (jstor.org) and that religion goes back thousands of years before Jesus of Nazareth declared his existence as the light in the world. Light and dark are associated with sight, and sight is associated with existence, understanding, love, and wisdom. That's why, "I see", is understood as "I understand".
From Encyclopedia.com's entry on "Light and Darkness":
LIGHT AND DARKNESS are basic natural phenomena as well as symbolic or metaphorical meanings that are often equated with the pairs of Being and Non-Being, primordial chaos and world order. According to the most ancient conceptions from the early civilizations of the Middle East, light and darkness are experienced in rhythmical alternation and hence as being contingent on each other. Darkness is the mysterious, impenetrable ground and source of light; and light becomes associated with creation. It grants and is therefore a symbol for the primal conditions of life: warmth, sensuality, and intellectual and spiritual enlightenment. In the course of the history of ideas, however, another concept was developed, in which darkness is an outcome of failure within the creational process.
Philosophies may tap in to basic, intuitive associations we make, which is why the symbolism they appropriate or explore often has a rich history outside of philosophy proper. Religions infamously explore certain moral and cosmogenical themes, and myth as presented by the mythologist Joseph Campbell and others, and the philosophy of religion (SEP) may delve into commonalities of religions, myth, and philosophy which has some degree of overlap. Carl Jung is famous among intellectuals for his notion of the collective unconscious:
According to Jung, the human collective unconscious is populated by instincts, as well as by archetypes: ancient primal symbols such as The Great Mother, the Wise Old Man, the Shadow, the Tower, Water, and the Tree of Life.6 Jung considered the collective unconscious to underpin and surround the unconscious mind, distinguishing it from the personal unconscious of Freudian psychoanalysis. He believed that the concept of the collective unconscious helps to explain why similar themes occur in mythologies around the world. He argued that the collective unconscious had a profound influence on the lives of individuals, who lived out its symbols and clothed them in meaning through their experiences. The psychotherapeutic practice of analytical psychology revolves around examining the patient's relationship to the collective unconscious.
Thus, one of the early philosophers of mind and certainly one of the most influential psychologists of all time attempted to explore universal themes and patterns in thought.
Today, coming from philosophers of language who posit that the mind functions largely by analogies below the surface of conscious introspection, much like the early school of psychoanalysts, cognitive semantics posits that there are conceptual metaphors:
A conceptual domain can be any mental organization of human experience. The regularity with which different languages employ the same metaphors, often perceptually based, has led to the hypothesis that the mapping between conceptual domains corresponds to neural mappings in the brain... This theory has gained wide attention, although some researchers question its empirical accuracy.
Thus, ideas like light and dark that start out in the visual domain may be associated below consciousness with more abstract concepts, such as fullness and emptiness or good and evil. Light-as-goodness and dark-as-evil are relatively common occurences in symbolism so much so that that general encyclopedia may have articles on it (Encyclopedia.com). What might explain this near universal persistence to these ideas? Well, in conceptual metaphor, there are positive associations of light, height, taste, and temperature that the brain is apt to associate with more abstract ideas like goodness, power, preference, and taste. These sorts of ideas are explored at in depth in Lakoff's The Metaphors We Live By which serve as the basis of his philosophy of embodied cognition, what he calls embodied realism.
It's a literal fact that human existence is dependent on light. We secure our calories ultimately from the sun (and a fair amount of vitamin D), so to equate light with survival is natural. Ancients were obsessed with what we know as astronomy because the seasons and light are intimately related; and whether or not you buy Campbell's work on the power of symbols in myth, Jung's collective unconscious, or Lakoff and Johnson's theses on conceptual metaphors, there certainly are a number of people who do and conduct research in these frameworks. One example of light and dark as symbols in modern time is this book by Charles Forceville, The GOOD IS LIGHT and BAD IS DARK Metaphor in Films which explores the interplay of metaphor and society.
It depends on what you mean by darkness. It has to do with what you see when it comes to darkness. But when talking about emptiness it has to do with the space that is around you. The difference between Darkness and Emptiness is that one has to do with sight and how you see it while one has to do with space and what is around you.