According to our current understanding of physics, light is considered to be massless, meaning it has a rest mass of exactly zero. This understanding is based on extensive experimental evidence and theoretical frameworks such as special relativity and quantum electrodynamics.
The concept of mass is a fundamental property of particles, and it affects how they interact with other particles and how they respond to forces. In the case of light, also known as electromagnetic radiation, it consists of particles called photons. Photons are the carriers of electromagnetic force and exhibit wave-particle duality.
Experimental measurements and observations have consistently shown that photons travel at the speed of light in a vacuum, which is a fundamental constant of nature denoted as 'c.' According to Einstein's theory of special relativity, particles with rest mass cannot reach or exceed the speed of light. Since photons do travel at the speed of light, it is concluded that they must have zero rest mass.
Furthermore, any hypothetical nonzero mass for the photon would introduce inconsistencies and contradictions with various established phenomena and theoretical frameworks. The properties of electromagnetic waves, including their behavior in vacuum and their interactions with charged particles, are successfully described by quantum electrodynamics and other theories that treat light as massless.
While it is always possible that future advancements in scientific knowledge could lead to revisions of our understanding, at present, the overwhelming consensus among physicists is that light is indeed massless.