I don't know. And I've studied quantum mechanics (the fundamentals - not the more advanced mathematics of it). I don't think anyone knows, though most people have their opinions. (I don't mean "anyone" in the strictest sense -- there are amazing people out there, and I wouldn't be surprised if a few of them somehow understand the fundamental nature of the universe.)

I feel like it probably doesn't make sense to say that light is only a wave, nor that it's only a particle. But it also doesn't seem right to say that light is both a wave and a particle. You could say that it's a wave or a particle depending on how you measure it, but that doesn't seem very reasonable either. All you can really say is that it appears as a wave or a particle depending on how you measure it, but that's not really satisfying -- what's behind those appearances? What causes those particular experimental results?

To understand this would basically be to understand quantum mechanics and what the "collapse of the wavefunction" really means / why it "happens." That's the million-dollar question.