It would be reasonable to assume that large galaxies of the early universe had enough 'fuel' to make new stars. But a recent discovery shows otherwise. Astronomers using the Hubble Space Telescope and the Atacama Large Millimeter/Submillimeter Array (ALMA) have found six early galaxies (about 3 billion years after the Big Bang) that were unusually "dead" that is, they'd run out of the cold hydrogen necessary for star formation. According to Kate Whitaker, the peak time for star births was this period. The disappearance of this hydrogen is therefore a mystery.
The strong gravitational lensing used galaxy clusters to magnify and bend light from the early universe helped the team find the galaxies. Hubble identified the locations of stars that had formed in the past. ALMA detected cold dust, which is a substitute for hydrogen. This was used to show the places stars could have formed if they had the right ingredients.
Galaxies have been expanding since then, but not by star creation. They grew from mergers with small galaxies and other gas. Any further formation would have been minimal.
These findings show the power of Hubble, ALMA and Hubble decades later. It also highlights the limitations of technology and human understanding, raising many questions. Whitaker pointed out that scientists aren't sure why galaxies die so quickly or how the fuel was cut off. Is the gas expelled, heated or consumed quickly? If answers are possible, it might take some time.