Dark matter is a name for whatever is causing extra gravity in galaxies and the universe that we cannot directly see with light. Astronomers infer it from how stars and galaxies move, not from a picture of the material itself.
The label does not mean scientists know exactly what it is. It means the gravitational evidence is strong enough that something additional is almost certainly there.
So the short answer is: it is invisible matter or matter-like stuff that helps explain why the universe behaves the way it does.
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