From my amateur understanding of space, it seems to be a galaxy made of just primordial hydrogen and helium, back before the first stars formed and started creating heavier elements due to the fusion reactions that power stars and the eventual supernovas that further dispersed and helped to create even more heavy elements.
Another cool theory is that the first stars are thought to have been much, much more massive, possibly up to around a thousand solar masses since they were made solely of hydrogen and helium. It’s estimated that current stars couldn’t get above a few hundred solar masses at most due to the existence of heavier elements in modern gas clouds. I don’t understand enough to explain why the existence of heavier elements limits star size so I’ll leave that to someone smarter than me.
Someone correct me if I got anything wrong, again I only have an amateur level of understanding about space.