When astronomers measure how fast stars orbit within a spiral galaxy, they find a result that ordinary physics does not predict. Stars near the visible edge move just as quickly as those farther in, even though the galaxy's visible matter thins out toward the rim. By the familiar laws of gravity, the outer stars should lag behind. Something appears to be holding them at high speed, and explaining what has split researchers into two main approaches.
The first and more widely held approach posits unseen matter. On this view, each galaxy is embedded in a vast halo of so-called dark matter, made of particles that emit no light but exert gravity. The extra mass supplies the pull needed to keep the outer stars moving fast. Supporters point out that the same hidden mass would also explain how clusters of galaxies hold together.
The second approach proposes instead that the law of gravity itself behaves differently at the very low accelerations found in a galaxy's outskirts. By modifying the law rather than adding matter, this account reproduces the flat orbital speeds without invoking any unseen particles. Critics counter that such a modification, while neat for single galaxies, struggles to explain larger structures that the dark-matter halo handles easily. No detection of a dark-matter particle has yet settled the contest.
Which of the following best states the main idea of the passage?
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