What 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.
What you will learn
- Say what dark matter and dark energy are suggested to explain
- Read a pattern and calculate a ratio from a table of space data
- Spot a result that does not fit and leave it out of a mean
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 locked.
In this lesson
- Mass we cannot see
- An expansion that is speeding up
- Reading space data
10 practice questions, marked as you go; about 20 minutes in all.
Key terms
Dark energy
In this unit: Space physics
The solar system and the Milky Way, the life cycle of a star, orbits and orbital speed, red-shift and the expanding universe, and what is still unknown.
- The solar system and the Milky Way
- The life cycle of a star
- Orbits and orbital speed
- Red-shift and the expanding universe
- Dark matter, dark energy and reading space data
- Models of the solar system and observing the universe