World 5 · The Missing Mass: Dark Matter · 12 min · PRO

Orbits in a galaxy: how Kepler predicts the speeds

The speed of an orbit tells you the mass inside it

PRO · Cosmos: Big Bang to Now

World 5: The Missing Mass: Dark Matter is part of PRO

The first world of Cosmos: Big Bang to Now is free. PRO opens the other 25 lessons in 6 worlds — simulations, quizzes and all.

Rubin's flat curves
More evidence: clusters, lensing and the Bullet
What could it be? And what we don't know
Galaxies are not alone: groups and clusters
Tidal tails and mergers

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