Although astronomers understand certain aspects of the universe clearly, others are more muddled. The Big Bang has a mountain of evidence behind it, while the picture of how protogalaxies evolved into normal galaxies in the first few billion years of the cosmos is just coming together. But what happened, exactly, at the end of the cosmic Dark Ages, following the Big Bang, that brought together the seeds of matter to form the first stars and galaxies? That question is still wrapped in a hazy cloud of mostly speculation.
Bringing the universe to your door. We’re excited to announce Astronomy magazine’s new Space and Beyond subscription box - a quarterly adventure, curated with an astronomy-themed collection in every box.
Learn More >>.
At the core of the issue is which came first: stars, galaxies, or black holes? With respect to galaxy formation, many astronomers believe in the “bottom-up” model of how matter came together. In this model, small clumps merged repeatedly to form protogalaxies, and further, that many small protogalaxies clumped together to form the larger, normal galaxies we see in the nearby universe today. But others believe giant sheets of matter formed in galaxy superclusters and then broke apart into smaller units. Either way, no one yet knows whether the gas and dust that came together to make galaxies preceded star formation or whether stars formed simultaneously as the first units of matter fell together to form the earliest protogalaxies.