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SESK: Stuff Everyone Should Know · sesk.info/astro-flat-rotation-curves

What do unexpectedly flat galaxy rotation curves imply?

They imply an extended halo of unseen mass.

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  1. Dark matter beyond visible stars ✔
  2. No gravity outside the disk
  3. Every outer star has equal mass

Why: Outer gas and stars orbit faster than visible matter alone permits. Extra gravity from an extended dark matter halo explains the high speeds.

Huh, didn't know that: A rotation curve plots orbital speed against distance from a galaxy's center.

The dad joke
The rotation curve stayed flat because dark matter had it covered.
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References

Answer: Dark matter beyond visible stars. Outer gas and stars orbit faster than visible matter alone permits. Extra gravity from an extended dark matter halo explains the high speeds.

  1. The Mass of the Galaxy openstax.org/books/astronomy-2e/pages/25-3-the-mass-of-the-galaxy
    the speed of gas clouds farther out remains high, indicating a great deal of mass beyond the Sun's orbit
    Checked 2026-10-10.

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