Why Galaxies Should Be Tearing Themselves Apart

The outer stars of this galaxy are moving so fast they should be flung into the void. So why isn't it falling apart?

A spiral galaxy can stretch a hundred thousand light years across and hold hundreds of billions of stars. By the visible matter alone, its edges spin far too quickly to stay bound.

In 1933, Fritz Zwicky studied the Coma cluster and found galaxies racing far faster than the visible mass could explain. He called the missing stuff dunkle Materie, dark matter. Decades later, in the 1970s, Vera Rubin and Kent Ford measured stars in spiral after spiral. The speeds should have dropped off toward the edges. Instead the curves stayed flat. Something unseen was holding everything together.

That missing mass may be roughly 85 percent of all matter in the universe. We still have not caught a single particle of it. We only see its gravity, an invisible scaffolding shaping everything we can see.

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