Over billions of years, the gas was compressed by gravity into galaxies, stars, plants and eventually, even humans.
在数十亿年的时间里，这些气体被引力压缩成星系、恒星、植物，最终甚至变成了人类。
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Meanwhile, its place is taken by hotter intergalactic gas from farther out on the edge of the galaxy, which cools as it is compressed and flows into the galaxy.