Ever since Democritus, the 5th and 4th BCE Greek philosopher, argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. 20th century breakthroughs showed that it behaves as a particle as well as a wave.
However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?