It is thought to account for around 27% of the universe, outweighing visible matter by a ratio of approximately six to one.
Everything in the universe that we can detect — all the atomic matter that makes up galaxies, stars, planets, life on Earth, the device that you’re reading this article on — is just a tiny, tiny fraction of all the matter that exists.
Dark matter is therefore theorized to be the secret ingredients that give these galaxies the extra mass they require to not tear themselves apart like a suicidal paper bag.
Weak gravitational lensing to the rescue
Looking for something that can’t be looked at sounds difficult.
They do this by looking at the subtle ways that light that the gravity of big galaxy clusters bends and distorts the light from more distant galaxies.