Ehh, its a bit more than that.
Its a particle in that we know they are quantized into single photons. As in, it is impossible to observe half of a photon, or any non-integer number of photons, and one photon can only be observed in one place. This makes it like a particle.
But its a wave in the way it behaves - it can interfere (not just with other photons, with itself), and its movement can only be described through wave functions that can even take seperate paths at the same time, according to how waves propogate.
And, there are ways in which they act like particles no matter how they are observed, and same for wavelike behavior
Worth noting: “observation” is just physical measurement. You have to keep in mind that observing something fundamentally requires interacting with it - in order to look at an apple, photons must bounce off of it, which is a physical interaction. On the quantum scale, these interactions cannot be ignored.
Also also: this isn’t just photons, everything is like this. It may not align with how we observe things on a macroscopic scale, but this is fundamentally how the universe works.
So the thing that gets weird is that the heavier the particle is the more likely it is to interact with the slits themselves on the way through, in which case the wavefunction will collapse and it will seem to go through only one slit. Also, as the other person stated, even a hydrogen atom is really 4 fundamental particles that can interact with eachother. I’m not totally sure if double slit has been demonstrated with atoms but I do know it’s been done many times with electrons.
Edit: its actually totally possible to do it with much, much larger things. From wikipedia:
And here’s the study that did it: https://doi.org/10.1038%2Fs41567-019-0663-9