As far as I know (I'm not a physicist), it's not all that clear that gravity is caused by a particle. It makes sense to assume it is, because of parallelisms with the other fundamental interactions, all of which (other than gravity) are caused by particles that have been thoroughly observed and studied.

So we kinda know what a graviton would be like and what to look for, but so far it hasn't been found, and its existence hasn't been conclusively determined. There are some alternative hypotheses that in fact gravitons don't exist at all, and gravity is just a consequence of the shape of space-time, which I think is what's going on with black holes.

(source: trust me bro I saw it on the Internet)