Between the distant quasars and the stars of the Sun's neighborhood lies a vast middle ground where the architecture of the cosmos stands in full display. Far from scattering at random as astronomers once surmised, galaxies tend to congregate, as people do. Moreover, their congregations also congregate. and so on. These vast clouds of organization--in which galaxies behave like swarming dust motes--have taught astronomers that there's much more to the universe than meets the eye. An astronomer can examine a galaxy and from its brightness make a guess as to how many stars and how much dusty gas it contains. Yet when we track the movements of galaxies in sef-contained cluster, they move as if each galaxy were far heavier than it appears. It's like watching go-carts move as unstoppably as trucks. What is this mass that looms invisibly among the bright lights of the galaxies? No one knows.
【正确答案】：在遥远的类星体和与太阳为邻的恒星之间有一片广阔的空间,在那里宇宙的结构得到了充分的展现。天文学家们一度猜测星系的分在是随机的，其实不然:众多星系和人一样有聚集的趋向,而且聚星体还会继续聚集。在这些庞大的星云体中,星系的性质就如同一颗颗密集的尘埃,它们使天文学家意识到真正的宇宙比我们肉眼所能看到的要大得多。尽管天文学家能够对某个星系进行观察并根据其亮度推测它所含恒星的数量和尘埃气体的多少,但是,当我们把众多星系作为一个独立存在的整体来观察其运动时就会发现,每个星系运动起来似乎比我们所看到的要重得多。这就像我们看一辆儿童手推车以卡车般巨大、难以阻挡的惯性向前奔驰一样可是,隐蔽在星系耀眼光芒中间的到底是什么物质呢?迄今仍无人知晓。
