Dark Matter Explained: The Invisible Stuff That Holds the Universe Together
What is dark matter? Explained simply — from the evidence for its existence to the leading theories about what it is. Daily astronomy and cosmology delivered to your inbox.
Dark matter is one of the great open mysteries of modern science. Astronomers have overwhelming evidence that about 27% of the universe is made of something that does not emit, absorb, or reflect light — but exerts gravitational effects we can measure. Galaxies rotate wrong if only visible matter is included. Galaxy clusters deflect light in ways that require far more mass than we can see. The universe's large-scale structure only makes sense if dark matter scaffolded the formation of galaxies. We know dark matter exists; we do not know what it is.
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What is dark matter in simple terms?
Dark matter is matter that doesn't interact with light — it doesn't emit it, reflect it, or absorb it — but does interact gravitationally. We infer its existence from the gravitational effects it has on visible matter and light. Galaxies rotate faster at their edges than they should if only visible matter were present; the extra gravitational pull must come from something invisible. Dark matter is not antimatter, black holes (in sufficient quantity), or a measurement error — it is something genuinely unknown.
What is the difference between dark matter and dark energy?
Dark matter and dark energy are completely different things. Dark matter is a form of matter that has mass and gravitational pull but does not interact with light. It acts to pull things together and is responsible for the large-scale structure of the universe. Dark energy is a property of space itself — a repulsive force driving the accelerating expansion of the universe. Dark matter makes up about 27% of the universe's total energy content; dark energy makes up about 68%; ordinary visible matter makes up only about 5%.