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)
Astronomical distances
What is a light year?
Dark energy
A mysterious process speeds up the Universe's expansion.
Dark matter
Invisible matter helps to hold the Universe together.
Extraterrestrial life
Is life out there?
General relativity
Einstein's gravity theory predicts warps in space-time.
Gravity
Newton lays the foundations of modern physics.
Hubble's Law
Hubble measures the Universe's expansion.
Quantum mechanics
There is a theory to explain the behaviour of very small things.
Special relativity
Einstein explains the behaviour of objects travelling very fast.
Spectroscopy
A powerful technique helps scientists 'fingerprint' starlight.
Steady State theory
Fred Hoyle puts forward an alternative to the Big Bang.
The Big Bang theory
The Universe begins in a huge expansion.
The Drake equation
How many advanced civilisations are in the Milky Way?
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