Astronomers find as many questions as they do answers. For example, in the 1970s Vera Rubin discovered that the stars at the edges of galaxies moved faster than expected. Her puzzling discovery led scientists to propose that invisible matter – dark matter – affected the stars' motion. Now the question is: What exactly is dark matter?
This section gathers together topics like dark matter that are unexplained – the questions – along with theories that have withstood a high level of scrutiny such as Einstein's theory of general relativity – the ideas.
Image: An illustration of two orbiting white dwarf stars creating gravitational waves in space (credit: NASA/GSFC/D. Berry)
What is a light year?
A mysterious process speeds up the Universe's expansion.
Invisible matter helps to hold the Universe together.
Is life out there?
Einstein's gravity theory predicts warps in space-time.
Newton lays the foundations of modern physics.
Hubble measures the Universe's expansion.
There is a theory to explain the behaviour of very small things.
Einstein explains the behaviour of objects travelling very fast.
A powerful technique helps scientists 'fingerprint' starlight.
Fred Hoyle puts forward an alternative to the Big Bang.
The Universe begins in a huge expansion.
How many advanced civilisations are in the Milky Way?
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