因为这些星系的质量分布在更大的区域，它们每单位体积的恒星数量比传统星系要少得多。
Because these galaxies' mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies.
每单位体积粉末的质量，包括粒子之间的空气。
Mass per unit of volume in powder form, including the air trapped between the particles.
在物理学上，是以每单位体积的质量来鉴别物质。
In physics it is an identification of material by noting its mass per unit volume.
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