Brainlag PlusWhat galaxy rotation and the speeding-up expansion suggest we cannot yet explain, and practice reading tables of space data for patterns, ratios and odd results.
Mass we cannot see
Stars far from the centre of a spiral galaxy should orbit more slowly than stars close in, just as Neptune is slower than Mercury.
Measurements show they do not: the outer stars move about as fast as the inner ones.
The extra gravity needed to hold them in orbit must come from mass we cannot see. Scientists call it dark matter: it does not give out or reflect light. Nobody yet knows what it is.
The rest of this lesson and its 10 practice questions are in Plus.
Screen 1 of 3, free to read.
- 10 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards