When you don’t understand something, sometimes the most human response is to ignore it. For decades, many scientists proved all too human. Astronomers looked to the distant universe and saw clusters containing thousands of galaxies. Each of these systems seems to be bound gravitationally, yet the visible material in the cluster members accounts for only 10 percent or so of the amount required to do the job. Closer to home, the outer regions of most spiral galaxies revolve faster than Newton’s laws predict based on the amount of material astronomers can see.
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