In physics it is an identification of material by noting its mass per unit volume.
在物理学上，是以每单位体积的质量来鉴别物质。
The mass per unit volume of a substance under specified conditions of pressure and temperature.
在特定压力和温度条件下某物质单位体积的质量。
Because these galaxies' mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies.
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。
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