What could be past the particle horizon?
It sounds like your question is about the single most important principle of cosmology, called the cosmological principle.  This asserts that as long as all our observations are consistent with the idea that the universe is the same everywhere at a given age, we will continue to use a model with that attribute.  There are lots of good reasons for adopting that principle, the least of which is not that it allows us to have a model in the first place.  So what this means is, our answer to what is beyond what we can see is that we don't know, but we are going to model it in a way that makes sense to us, so we will use the cosmological principle.   You have to be able to distinguish what we know in the sense of having direct information about, versus what we know about our own best models.