  The discovery of Fast Radio Bursts (FRBs) at cosmological distances has
opened a powerful window on otherwise unseen matter in the Universe. In the
2020s, observations of $>10^{4}$ FRBs will assess the baryon contents and
physical conditions in the hot/diffuse circumgalactic, intracluster, and
intergalactic medium, and test extant compact-object dark matter models.
