A Systematic Literature Review of the Scientific Hypotheses Explaining the Wow! Signal (1977–2026)

The Wow! Signal, a 72-second-long narrowband radio emission detected by the Ohio State University “Big Ear” radio telescope on 15 August 1977, is the single most investigated unidentified event in the history of SETI. Despite extensive efforts, no other similar signals were detected again since, and no hypothesis could reach scientific consensus. In this work, I review and analyze the most plausible theories proposed in 1977-2026 (both published and unpublished) to explain the origin of the detected radio emission. Among these, the hypothesis of artificial extraterrestrial intelligence, the stimulated maser-type amplification of the cold interstellar medium hydrogen line emission, the cometary hydrogen radiation flash, and terrestrial interference were found to be the most plausible. The review was conducted following the inclusion/exclusion criteria, and after the screening stage, twenty-plus studies were included in the final analysis. Based on the data extracted from these publications, I argue that the most probable causes of the Wow! Signal are either a natural interstellar medium astrophysical phenomenon (which is supported by the 2024-2025 reanalysis of the original Big Ear telescope data) or some unrepeated artificial radio emission. While the latter is formally impossible to rule out, its likelihood is significantly reduced due to the prolonged observational campaigns’ outcomes. In this review, I detail the evidence in favour of these arguments, outline some critical research gaps, and identify the critical future steps necessary to address them.