Almost everything widely known about Dogon cosmology, the Nommo included, comes from the fieldwork of the French anthropologists Marcel Griaule and Germaine Dieterlen, whose books, above all Griaule's Dieu d'eau (Conversations with Ogotemmeli, 1948), presented an extraordinarily elaborate and systematic Dogon cosmology to the world and made the Dogon among the most famous peoples in the anthropological literature. That account is also, today, seriously contested.
The anthropologist Walter van Beek, who carried out later fieldwork among the Dogon, reported that he could not find the elaborate esoteric cosmology Griaule described, and argued that it may have been shaped, perhaps unintentionally, by the questions Griaule pressed and by the answers of a small number of informants, so that the famous system may reflect the encounter between ethnographer and informant as much as a settled body of Dogon doctrine. (A separate and far more extravagant claim sometimes attached to this material, that the Dogon possessed secret ancient knowledge of the invisible companion star of Sirius, is regarded by scholars as fringe speculation, not sober ethnography, and is best set aside entirely.)
None of this means the Nommo are an invention; that the Dogon honour primordial water-beings associated with Amma, creation and the fertilising rains is not in doubt. It means that the precise, encyclopaedic detail of the classic accounts should be treated as a contested reconstruction rather than as simple fact. The Dogon are a living people with a rich religious life of their own, and the honest report is this: the Nommo are their ancestral water-spirits, bringers of order and of the word, at the heart of a cosmology whose famous elaboration rests on a body of fieldwork that later scholars have called into question.