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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The “really” part of the question is the kicker. Particles are not truly particles, is pretty much the only answer. There are entities which behave the same way a particle would be expected to behave in certain specific situations so the entity is called a “particle” for the purposes of the situation. In other conditions, the very same thing might behave like a wave and thus not be called a particle for that situation.
The very term “particle” is a convenience for us humans. In an ideal universe, a particle would be a uniquely-sized bit of matter occupying a very specific volume of space at some instant in time (so we can disregard its motion when considering its location), and even more ideally, would be spherical. Such things do not exactly and “really” exist, but we have to start somewhere if we want to try to identify the rules of reality which govern its behavior, and as long as the entity behaves pretty much like such an ideal object would behave, then we speak of it as a “particle”. But it is not correct to actually imagine that it is an idealized particle, or even a less than ideally-shaped particle.
When you are deally with very tiny and energetic “things”, they don’t really have a specific volume of space which they occupy even at a frozen instant of time. It is more of a waveform that attenuates outward so eventually it may as well not exist. It is not really possible to define a limit, a boundary in space, where the thing goes from presence to non-existence, which is, in itself, a basic requirement to call a thing a particle in a real sense. So it is not, really and truly, a particle. It just behaves like one for certain situations.
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