An Assiti Shard is a piece of artwork created by the Assiti. This artwork is a convoluted piece of spacetime that causes transpositions of volumes of spacetime with other pieces of spacetime, the Assiti Shards effect. The Assiti are hated for the impact that their artwork has on other races, and are ultimately exterminated by the Fta Tei for their unwillingness to control their artwork's impact on other races.
Assiti Shards do affect instruments such as neutrino detectors, at least to some degree. They appear to have a distinct "signature" which can be analyzed in ways that allow their path to be modeled and their intensity to be estimated.