The role of dark matter halos
So how can we explain the findings? Well, we know that there was a lot of gas present in these early galaxies, constantly flowing in from the intergalactic medium. These gas reservoirs make ordinary matter effectively sink to the centres of the dark matter halos that host them, piling up.
It could also be that during these early times the dark matter halos were growing rapidly and were not yet in equilibrium. This means that the odds that galaxies could form in regions of lower dark matter concentration were higher.
Cosmic time scales are long. Mapping the evolutionary paths of galaxies throughout the history of the universe requires piecing together snapshots of their lives, as observed at different epochs. Undoubtedly, our new findings will add a valuable piece to this puzzle. What we can say for now is that the disk galaxies we observed three billion years after the Big Bang are markedly different compared to Milky Way type galaxies today.
But it’s important to remember that when comparing these ancient galaxies to ones ten billion years later, one should also take into account that new stars will be formed in the meantime. In the search for descendant galaxies, it therefore seems more relevant to look at modern galaxies that are more massive than the Milky Way. Those are often spheroidal in shape (they lack spiral arms). Interestingly, their dynamics also point at low dark matter concentrations.
Looking ahead, we want to uncover the physics behind such evolution, and explore how our findings can inform the theory on how normal and dark matter interact. Perhaps it could even help us to answer the biggest question of all: what dark matter really is.