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When the cosmos was between 2 and 4 billion years old galaxies were just cranking along, converting huge reservoirs of cold gas into stars at a fierce rate, some easily 100 times what we tend to see today in our 13.8-billion-year-old Universe. Because light takes time to get from them to us, we see these galaxies as being 9–12 billion or so light years from us, and almost all the big ones we see at that distance are the proud parents of huge litters of stars. This was a time of peak star formation in the Universe, and in fact astronomers call it “cosmic noon”.
But — and this is a big but — at that distance the bigger galaxies tend to be luminous, the ones making the most stars. Ones with lower star birth rates are much harder to see, so we may be suffering what’s called an observational bias, or a selection effect. Those faint galaxies might be there, but we can’t see them.
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