What really is a particle? How can massless particles exist? How can it still be a particle if it doesn’t have any mass?
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That is by far not an ELI5 question. Most people would struggle with it as an ELI25.
I would recommend the PBS Spacetime show on Youtube, they have series of videos on this topic, and are good at explaining it as concisely as it is possible to explain quantum physics.
But to give it a shot, particles are vibrations in quantum fields, which are currently the actual fundamental building blocks of reality, and by default should NOT have any mass. Being massless is actually a particle’s natural state. Most elementary particles gain mass due to the interaction of their fields with the Higgs Field (of Higgs Boson fame), while composite particles (hadrons) gain most of their mass from the energy of their gluon bonds (because E=mc^2).
Why some particles interact with the higgs field and some don’t is literally a PhD topic.
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