For several decades after their discovery, the origin of these events remained enshrouded in mystery. The largest limitation in our understanding was the uncertainty on the
distance scale. GRB astronomers even held a "
Great Debate" in 1995, in the model of the Shapley-Curtis Debate in 1920, to consider arguments either for a Galactic or extragalactic origin.
Strong circumstantial evidence in favor of an extragalactic origin came from the first large population of GRBs from the BATSE instrument on-board the Compton Gamma-Ray Observatory. While each individual GRB was poorly localized on the sky (error regions of tens to hundreds of square degrees), the entire population of thousands of events was clearly distributed isotropically on the sky (Figure 2). Given the lack of correlation with the stellar population of the Milky Way (i.e., the Galactic plane), an extragalactic origin for GRBs was strongly favored.