Given a nearly infinite timeline, would the expansion of space eventually cause individual particles to become "unobservable" from one another due to the vast distance and increasing rate of expansion with that distance.
Maybe someone can give chapter and verse on this, but my understanding is that bound objects are unaffected by the expansion for the case of dark energy represented by a simple cosmological constant.
But a "big rip" will occur in a finite time if the dark energy equation of state $P = w \rho$ is characterised by $w < -1$. In this scenario, the expansion accelerates so quickly that the scale factor becomes infinite in a finite time and structures must disintegrate when the size of the observable universe becomes smaller than that structure.
The present state of play is that $w$ appears to be very close to -1 (a cosmological constant). But given that dark energy is not understood, who can really say whether it might change in the future...