The background universe is a comprehensive set that represents all possible genes, proteins that could have been detected in your experiment. This universe is crucial for determining the statistical likelihood that the observed enrichment in your test set is due to chance. Your file (.txt or .csv) should have one ID per row, similar to gene list upload and use same ID type as your uploaded list(s). If no universe is supplied, the background universe is defined by the gene set/pathway library selected when performing analysis.