What is the nature of 'Oumuamua?

1I/'Oumuamua, the first known interstellar visitor (2017), showed non-gravitational acceleration without visible outgassing — unlike any known comet. Its highly elongated (cigar- or pancake-like) shape inferred from the light curve is also unusual.

Proposed explanations include a 'cosmic iceberg' of frozen hydrogen, a nitrogen-ice fragment, or other exotic compositions. Future interstellar objects (2I/Borisov, 3I/ATLAS) may help settle the picture.